doc: update fast-track approval comment request

Collaborators no longer need to add a comment for other collaborators
to upvote after adding the `fast-track` label as the GitHub actions
bot is configured to automatically add a comment after the label is
added.

doc/guides/collaborator-guide.md

@@ -184,8 +184,8 @@ delay. For example:
   * Regressions that break the workflow (red CI or broken compilation).
   * Regressions that happen right before a release, or reported soon after.
 
-To propose fast-tracking a pull request, apply the `fast-track` label. Then add
-a comment that collaborators can upvote.
+To propose fast-tracking a pull request, apply the `fast-track` label. Then the
+GitHub actions bot will add a comment that collaborators can upvote.
 
 If someone disagrees with the fast-tracking request, remove the label. Do not
 fast-track the pull request in that case.