docs (build_ast_schema): add Subscription to list of root types

Replicates graphql/graphql-js@91bc70b

Author: Cito

src/graphql/utilities/build_ast_schema.py

@@ -20,8 +20,8 @@ def build_ast_schema(
     This takes the ast of a schema document produced by the parse function in
     src/language/parser.py.
 
-    If no schema definition is provided, then it will look for types named Query
-    and Mutation.
+    If no schema definition is provided, then it will look for types named Query,
+    Mutation and Subscription.
 
     Given that AST it constructs a GraphQLSchema. The resulting schema has no
     resolve methods, so execution will use default resolvers.