Update docs feedback IRC channel to -doc

This channel didn't exist until relatively recently. Also mention
freenode explicitly.

index.rst

@@ -28,7 +28,7 @@ in the top left corner.
 
           Submit an issue or pull request on the `GitHub repository
           <https://github.com/godotengine/godot-docs/issues>`_, or discuss with
-          us on the #godotengine-devel IRC channel!
+          us on the #godotengine-doc IRC channel on freenode!
 
 The main documentation for the site is organized into the following sections: