Fix error in PostgreSQL enum example. (#296)

As per the [PostgreSQL documentation](https://www.postgresql.org/docs/13/sql-createtype.html), the keyword `ENUM` is required when creating an enum type.

Author: William Hart

src/pages/postgraphile/enums.md

@@ -9,7 +9,7 @@ be automatically renamed in order to make sure they conform to the GraphQL
 naming requirements and conventions.
 
 ```sql
-create type animal_type as (
+create type animal_type as enum (
   'CAT',
   'DOG',
   'FISH'