Fix GraphQL example schema construction

[funwithcthulhu]

Summary

• Build the GraphQL object and schema through functions so the example does not keep weak type variables at top level.
• Apply the same shape to the Reason example and keep both README snippets in sync.

Fixes #373.

Validation

• git diff --check

Blocked locally:

• opam exec -- dune build example/i-graphql/graphql.exe needs graphql-lwt, which is not installed in this switch.