docs: fix GraphQL photo author schema

[oiahoon]

Summary

• update the documented GraphQL Photo schema to expose author: Author
• update the shop query example to select the nested author fields

Root cause

The README still documented the internal authorId field, while the public GraphQL schema resolves that value to an Author object before returning a photo.

Validation

• confirmed the previous example returns Cannot query field "authorId" on type "Photo" from the public endpoint
• confirmed the updated query returns the nested author object from https://ramen-api.dev/graphql
• compared the example with src/graphql.ts and the existing GraphQL test query
• git diff --check

Fixes #13

[yusukebe]

LGTM!

[yusukebe]

@oiahoon

Nice! Looks good!