GraphQL tutorial uses __type() without defining it

[damonfeldman]

Modify this page https://dgraph.io/tour/graphqlbasic/3/

to say that __type() is an automatically generated field of the required __Type type, per the GraphQL spec.

Extend this text

"The GraphQL schema tells us how to interpret every field. Run the example introspection query and have a look at the JSON result."

to

"The GraphQL schema includes introspection fields and parameters including a __Type type with a __type() root-level field. Run the example introspection query and have a look at the JSON result."

If possible, make "introspection fields and parameters' hyperlink to https://graphql-ruby.org/schema/introspection .