Docs: type of `Response` missing the second type var

[mattibo]

Summary

With the change in #173 the type of response has changed.

The README and the documentation were not updated.

This part

githubkit/README.md

Line 99 in 8f44dc0

resp: Response[FullRepository] = github.rest.repos.get("owner", "repo")

should look like

resp: Response[FullRepository, FullRepositoryType] = github.rest.repos.get("owner", "repo") 

Affected files

• README.md
• docs/usage/rest-api.md
• docs/usage/unit-test.md

[yanyongyu]

The change is actually in #158 . The second type var of the Response type is optional and only takes effects when using response.json(). It is Any by default. In the docs, the code is for readers to inspect the type only. But the example for response.json() uses the wrong type, I will fix it.