doc: fix switched rows in board selection guide

The nRF52840DK is an excellent wireless dev board due to the
integrated debugger, the nRF52840-Dongle is missing the debugger and
therefore a suboptimal choice.

The nRF52840-Dongle is cheap and has an excellent form factor that
allows to easily plug it into e.g. an OpenWRT system to quickly provide
a border router. The nRF52840DK is too large and too expensive for that
use case.

It appears that the rows have just been switched by mistake, this
switches them back to be correct.

boards/doc.txt

@@ -54,8 +54,8 @@ Popular Boards
 | @ref boards_esp32_esp-ethernet-kit    | Development           | `+++`         | ✔                     | ✔ (WiFi, BLE, Ethernet, custom)   | ✗             | ✗                     | Custom                    | `-` (1 button)                                        | `+`                   | `o`       | Requires proprietary software |
 | @ref boards_esp32_wroom-32            | Prototyping           | `+++`         | ✗                     | ✔ (WiFi, BLE, custom)             | ✗             | ✗                     | Custom                    | `-` (1 button)                                        | `+++`                 | `+++`     | Requires proprietary software |
 | @ref boards_microbit_v2               | Education             | `++`          | ✔                     | ✔ (802.15.4, BLE, custom)         | ✗             | ✗                     | micro:bit edge connector  | `+++` (6 buttons, LED matrix, mic, speaker, IMO)      | `--`                  | `++`      | Good education board          |
-| @ref boards_nrf52840dk                | Development           | `++`          | ✔                     | ✔ (802.15.4, BLE, custom)         | ✔             | `+++`(Uno, Mega)      | Custom                    | `+` (4 buttons, 4 LEDs)                               | `+`                   | `+`       | Excellent border router       |
-| @ref boards_nrf52840dongle            | Prototyping           | `++`          | ✗                     | ✔ (802.15.4, BLE, custom)         | ✔             | ✗                     | Custom                    |  o  (1 button, 4 LEDs)                                | `++`                  | `++`      | Good wireless dev board       |
+| @ref boards_nrf52840dk                | Development           | `++`          | ✔                     | ✔ (802.15.4, BLE, custom)         | ✔             | `+++`(Uno, Mega)      | Custom                    | `+` (4 buttons, 4 LEDs)                               | `+`                   | `+`       | Good wireless dev board       |
+| @ref boards_nrf52840dongle            | Prototyping           | `++`          | ✗                     | ✔ (802.15.4, BLE, custom)         | ✔             | ✗                     | Custom                    |  o  (1 button, 4 LEDs)                                | `++`                  | `++`      | Excellent border router       |
 | @ref boards_nucleo-wl55jc             | Development           | `++`          | ✔                     | ✔ (LoRa)                          | ✗             | `+` (Uno)             | ST Morpho Headers         | `+` (3 buttons, 3 LEDs)                               | `+`                   | `++`      | Good bang for the buck        |
 | @ref boards_pinetime                  | Gadget                | `++`          | ✗                     | ✔ (BLE)                           | ✗             | ✗                     | ✗                         | `+++` (LCD, button, touch screen, IMU, flash, ...)    | `---`                 | `+++`     | Buy two: One with SWD access  |
 | @ref boards_samr21-xpro               | Development           | `+`           | ✔                     | ✔ (802.15.4)                      | ✔             | ✗                     | XPRO Expansion Header     | `o` (1 button, 1 LED)                                 | `+`                   | `--`      | Quite expensive               |