Fix spelling

Replicates graphql/graphql-js@735b43d

Author: Cito

tests/utilities/test_build_client_schema.py

@@ -327,7 +327,7 @@ def builds_a_schema_with_field_arguments():
 
               """A field with a two args"""
               two(
-                """This is an list of int arg"""
+                """This is a list of int arg"""
                 listArg: [Int]
 
                 """This is a required arg"""