Change to '--no-update-readme' in commit message

After the change to add '--no' to --update-readme (swapping the default
behaviour), I forgot to change the commit message.

E.g:
* https://crrev.com/c/6558948
* https://crrev.com/c/6553930

Change-Id: I5f68e6b0517a0a9ad7182c7389332134c98c0c0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6559761
Reviewed-by: Rachael Newitt <renewitt@google.com>
Reviewed-by: Scott Lee <ddoman@chromium.org>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Auto-Submit: Jordan Brown <rop@google.com>

roll_dep.py

@@ -380,8 +380,8 @@ def main():
         d.replace('\\', '/').rstrip('/') for d in args.dep_path)
     cmdline = 'roll-dep ' + ' '.join(dependencies) + ''.join(' --key ' + k
                                                              for k in args.key)
-    if not args.no_update_readme:
-        cmdline += ' --update-readme'
+    if args.no_update_readme:
+        cmdline += ' --no-update-readme'
     try:
         if not args.ignore_dirty_tree and not is_pristine(current_dir):
             raise Error('Ensure %s is clean first (no non-merged commits).' %