Clarify options that control the repo executable version

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/repo b/repo
index 7498d09..176c563 100755
--- a/repo
+++ b/repo
@@ -120,7 +120,7 @@
                  help='mirror the forrest')
 
 # Tool
-group = init_optparse.add_option_group('Version options')
+group = init_optparse.add_option_group('repo Version options')
 group.add_option('--repo-url',
                  dest='repo_url',
                  help='repo repository location', metavar='URL')