project: fix invalid logger call

This used to be a print() call, but the migration was incomplete.

Change-Id: If17e5108aa3d2a2948c4834e9c6ed895d3809f91
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/656724
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>

project.py

@@ -6331,7 +6331,7 @@ class ManifestProject(MetaProject):
         elif platform in all_platforms:
             groups.append(platformize(platform))
         elif platform != "none":
-            logger.error("fatal: invalid platform flag", file=sys.stderr)
+            logger.error("fatal: invalid platform flag: %s", platform)
             return False
         self.config.SetString("manifest.platform", platform)