Use different metavar for --filter-by-project instead of --filter-by-tag

I just realized `--filter-by-project` is using the same metavar as `--filter-by-tag`. Most likely a copy-paste error.

Author: ahoppen

run_sk_stress_test

@@ -67,7 +67,7 @@ def parse_args():
                         help='Only run project actions with the given tag',
                         default='sourcekit')
     parser.add_argument('--filter-by-project',
-                        metavar='TAG',
+                        metavar='PROJECT',
                         help='Only stress test the given project')
     parser.add_argument('--sourcekit-xfails',
                         metavar='PATH',