docs: fix example given for GraphQL::Schema::Enum

The example inherited from `GraphQL::Enum` - a constant
that doesn't exist. Update the example to inherit from
`GraphQL::Schema::Enum` instead.

Author: 0x1eef

lib/graphql/schema/enum.rb

@@ -14,7 +14,7 @@ class Schema
     #   #   ONIONS
     #   #   PEPPERS
     #   # }
-    #   class PizzaTopping < GraphQL::Enum
+    #   class PizzaTopping < GraphQL::Schema::Enum
     #     value :MUSHROOMS
     #     value :ONIONS
     #     value :PEPPERS