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/genqlient.yaml

@@ -1,10 +1,11 @@
-# These are the defaults, and are just included to be explicit.
-package: example
 schema: schema.graphql
 operations:
 - genqlient.graphql
 generated: generated.go
 
+# needed since it doesn't match the directory name:
+package: main
+
 # We bind github's DateTime scalar type to Go's time.Time (which conveniently
 # already defines MarshalJSON and UnmarshalJSON).  This means genqlient will
 # use time.Time when a query requests a DateTime, and is required for custom