github graphql schema location changed

[jchidley]

This comment:

//! Run this to update `github_schema.graphql`:
//!
//! ```sh
//! curl -L https://docs.github.com/public/schema.docs.graphql -o examples/github_schema.graphql
//! ```

at the top of examples/graphql_issues.rs

needs changing to

//! Run this to update `github_schema.graphql`:
//!
//! ```sh
//! curl -L https://docs.github.com/public/fpt/schema.docs.graphql -o examples/github_schema.graphql
//! ```

Because the location for Github's GraphQL Schema appears to have changed.