Clean up example a little (#89)

## Summary:
I realized:
- it's simpler to avoid the usual `cmd` and just put it all in the same
  directory
- we don't need schema.json since GitHub now posts a schema in SDL
  format

## Test plan:
make lint example


Author: benjaminjkraft

Reviewers: dnerdy, StevenACoffman, aberkan, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: dnerdy, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/89

example/README.md

@@ -6,7 +6,7 @@ Create a [personal access token](https://docs.github.com/en/github/authenticatin
 To run the example:
 
 ```sh
-$ GITHUB_TOKEN=<your token> go run ./cmd/example/main.go <username>
+$ GITHUB_TOKEN=<your token> go run . <username>
 you are Ben Kraft
 csilvers is Craig Silverstein
 ```