docs: minor CONTRIBUTING.md fixes
docs: minor CONTRIBUTING.md fixes

Change-Id: I6e649fc53313f775ea04df7a0022e695a9617d3f
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/46975
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>

googleapis/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,google/google-api-go-client