fix missing quote in --key error message (#5344)

CHANGES.rst

@@ -4,6 +4,7 @@ Version 3.0.1
 Unreleased
 
 -   Correct type for ``path`` argument to ``send_file``. :issue:`5230`
+-   Fix a typo in an error message for the ``flask run --key`` option. :pr:`5344`
 
 
 Version 3.0.0

src/flask/cli.py

@@ -795,7 +795,9 @@ def _validate_key(ctx, param, value):
 
         if is_context:
             raise click.BadParameter(
-                'When "--cert" is an SSLContext object, "--key is not used.', ctx, param
+                'When "--cert" is an SSLContext object, "--key" is not used.',
+                ctx,
+                param,
             )
 
         if not cert: