Fixing some broken relative links. (#496)

Because of a quirk in the way URL paths are generated, if you're making
a relative link from `index.mdx`, you specify it as:

```markdown
[text](./relative.mdx)
```

But if it's *not* in the index page - if it's a relative link to a
regular page in the same directory - you need to use a double-dot:

```markdown
[text](../relative.mdx)
```

This commit fixes three such broken links.

docs/tutorials/misc/c-service.mdx

@@ -21,7 +21,7 @@ send byte arrays back and forth.
 ## Prerequisites
 C services need to be compiled into the native firmware. As such, we need to
 build a custom envelope. Make sure to have finished the
-[custom envelope tutorial](./custom-envelope) before continuing.
+[custom envelope tutorial](../custom-envelope) before continuing.
 
 ## The C service
 The [template repository](https://github.com/toitlang/template-custom-envelope)

docs/tutorials/misc/custom-envelope.mdx

@@ -54,7 +54,7 @@ the template to match your needs. For this tutorial we assume that you are
 using an ESP32 (the default), and that you will modify license, and the CI
 action at a later point. We will also keep the echo-service in the components
 directory, for now. You should remove it if you don't need it. See
-the [c-service tutorial](./c-service) for more information.
+the [c-service tutorial](../c-service) for more information.
 
 Update the toit SDK to your preferred version, and then initialize the submodules.
 The SDK version you want depends on your needs. If you want to use

docs/tutorials/network/cellular.mdx

@@ -28,7 +28,7 @@ In later sections we will refer to [containers](../containers) and
 [services](../containers/services), but these are not strictly necessary.
 
 We will use HTTP to fetch data from the internet. There are
-[multiple](./http) [tutorials](./http-server) on this topic, but you
+[multiple](../http) [tutorials](../http-server) on this topic, but you
 should be able to follow this tutorial without having read them.
 
 ## Packages