fix: missing f-string in App._parse_action()

Author: tfh-cri

src/textual/app.py

@@ -3678,7 +3678,7 @@ def _parse_action(
                 raise ActionError(f"Action namespace {destination} is not known")
             action_target = getattr(self, destination, None)
             if action_target is None:
-                raise ActionError("Action target {destination!r} not available")
+                raise ActionError(f"Action target {destination!r} not available")
         return (
             (default_namespace if action_target is None else action_target),
             action_name,