Add cmd/ctrl-s keyboard shortcuts for file editor (#18535)

* Add cmd/ctrl-s for file editor

* Update content/github/getting-started-with-github/keyboard-shortcuts.md

Author: JasonEtco

content/github/getting-started-with-github/keyboard-shortcuts.md

@@ -52,6 +52,7 @@ Below is a list of some of the available keyboard shortcuts.
 |<kbd>control z</kbd> or <kbd>command z</kbd> | Undo
 |<kbd>control y</kbd> or <kbd>command y</kbd> | Redo
 |<kbd>cmd + shift + p</kbd> | Toggles between the **Edit file** and **Preview changes** tabs
+|<kbd>control s</kbd> or <kbd>command s</kbd> | Write a commit message
 
 For more keyboard shortcuts, see the [CodeMirror documentation](https://codemirror.net/doc/manual.html#commands).