docs: replace CL with PR in CONTRIBUTING.md
Fixes:
1. [File an issue](https://github.com/googleapis/google-cloud-go/issues/new/choose).
The issue will be used to discuss the bug or feature and should be created
- before sending a CL.
+ before sending a PR.
1. [Install Go](https://golang.org/dl/).
1. Ensure that your `GOBIN` directory (by default `$(go env GOPATH)/bin`)
