update --help

Signed-off-by: William Woodruff <william@trailofbits.com>

Author: woodruffw

README.md

@@ -108,6 +108,7 @@ positional arguments:
     get-identity-token
                        retrieve and return a Sigstore-compatible OpenID
                        Connect token
+    plumbing           developer-only plumbing operations
 
 optional arguments:
   -h, --help           show this help message and exit

sigstore/_cli.py

@@ -424,7 +424,7 @@ def _parser() -> argparse.ArgumentParser:
     # `sigstore plumbing`
     plumbing = subcommands.add_parser(
         "plumbing",
-        help="Developer-only plumbing operations",
+        help="developer-only plumbing operations",
         formatter_class=argparse.ArgumentDefaultsHelpFormatter,
         parents=[parent_parser],
     )