Avoid src being marked as optional in help
diff --git a/src/black/__init__.py b/src/black/__init__.py
index 8e2123d50cc..51384fb08da 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -328,6 +328,7 @@ def validate_regex(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,
+    metavar="SRC ...",
 )
 @click.option(
     "--config",

https://github.com/psf/black/issues/704#issuecomment-490251352

I *hope* this is trivial enough where it doesn't need a changelog entry (mostly because I forgot about it and am too lazy to add one).