Update basic-writing-and-formatting-syntax.md

add single tilde `~` as valid strikethrough syntax

Author: amitse

content/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md

@@ -36,7 +36,7 @@ You can indicate emphasis with bold, italic, strikethrough, subscript, or supers
 | --- | --- | --- | --- | --- |
 | Bold | `** **` or `__ __`| <kbd>Command</kbd>+<kbd>B</kbd> (Mac) or <kbd>Ctrl</kbd>+<kbd>B</kbd> (Windows/Linux) | `**This is bold text**` | **This is bold text** |
 | Italic | `* *` or `_ _`     | <kbd>Command</kbd>+<kbd>I</kbd> (Mac) or <kbd>Ctrl</kbd>+<kbd>I</kbd> (Windows/Linux) | `_This text is italicized_` | _This text is italicized_ |
-| Strikethrough | `~~ ~~` | None | `~~This was mistaken text~~` | ~~This was mistaken text~~ |
+| Strikethrough | `~~ ~~` or `~ ~` | None | `~~This was mistaken text~~` | ~~This was mistaken text~~ |
 | Bold and nested italic | `** **` and `_ _` | None | `**This text is _extremely_ important**` | **This text is _extremely_ important** |
 | All bold and italic | `*** ***` | None | `***All this text is important***` | ***All this text is important*** | <!-- markdownlint-disable-line emphasis-style -->
 | Subscript | `<sub> </sub>` | None | `This is a <sub>subscript</sub> text` | This is a <sub>subscript</sub> text |