Add 'about' to group links so the description shows (#1958)

CONTRIBUTING.md

@@ -169,11 +169,11 @@ access. All requests require a
 [signed CLA](#contributor-license-agreements-clas).
 
 -   Google Docs/Calendar commenter access:
-    [join group](https://groups.google.com/a/carbon-lang.dev/g/commenters)
+    [join group](https://groups.google.com/a/carbon-lang.dev/g/commenters/about)
     -   Grants commenter access to Carbon Language documents, as well as the
         ability to view Calendar events.
 -   Google Docs/Calendar contributor access:
-    [join group](https://groups.google.com/a/carbon-lang.dev/g/contributors)
+    [join group](https://groups.google.com/a/carbon-lang.dev/g/contributors/about)
     -   Grants edit access to Carbon Language documents, as well as the ability
         to edit Calendar events.
 -   GitHub Label/project contributor access: