Replace 'A new repo command' with 'A new version of repo'

Change-Id: I3288f5c963b69d05d113fc039e4b4f22721f1de9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254667
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>