Fix grammar in error message

Replicates graphql/graphql-js@2daa0ed

Author: Cito

graphql/type/schema.py

@@ -232,7 +232,7 @@ def type_map_reducer(map_: TypeMap, type_: GraphQLNamedType = None) -> TypeMap:
     if name in map_:
         if map_[name] is not type_:
             raise TypeError(
-                "Schema must contain unique named types but contains multiple"
+                "Schema must contain uniquely named types but contains multiple"
                 f" types named {name!r}."
             )
         return map_

tests/type/test_schema.py

@@ -223,7 +223,7 @@ def rejects_a_schema_which_redefines_a_built_in_type():
                 GraphQLSchema(QueryType)
             msg = str(exc_info.value)
             assert msg == (
-                "Schema must contain unique named types"
+                "Schema must contain uniquely named types"
                 f" but contains multiple types named 'String'."
             )
 
@@ -237,7 +237,7 @@ def rejects_a_schema_which_defines_an_object_twice():
                 GraphQLSchema(types=types)
             msg = str(exc_info.value)
             assert msg == (
-                "Schema must contain unique named types"
+                "Schema must contain uniquely named types"
                 f" but contains multiple types named 'SameName'."
             )
 
@@ -255,7 +255,7 @@ def rejects_a_schema_which_defines_fields_with_conflicting_types():
                 GraphQLSchema(QueryType)
             msg = str(exc_info.value)
             assert msg == (
-                "Schema must contain unique named types"
+                "Schema must contain uniquely named types"
                 f" but contains multiple types named 'SameName'."
             )