Fix misleading test section description

Replicates graphql/graphql-js@c994728

Author: Cito

tests/validation/test_defer_stream_directive_label.py

@@ -9,7 +9,7 @@
 assert_valid = partial(assert_errors, errors=[])
 
 
-def describe_defer_stream_label():
+def describe_defer_stream_directive_labels():
     def defer_fragments_with_no_label():
         assert_valid(
             """