shellcheck_run_steps: Use an argparse metavar as help text

I think this explains the whole -- business more clearly.

Signed-off-by: dann frazier <dann.frazier@chainguard.dev>

Author: dannf

pre_commit_hooks/shellcheck_run_steps.py

@@ -71,8 +71,7 @@ def main(argv: Sequence[str] | None = None) -> int:
     parser.add_argument(
         "filenames",
         nargs="*",
-        help="Filenames to check. You can also pass "
-        "arguments to shellcheck before a '--' separator.",
+        metavar="[-- SHELLCHECK ARGS -- ] FILENAMES",
     )
     parser.add_argument(
         "--shellcheck",