cloud: Fix CONTRIBUTING.md instructions.
cloud: Fix CONTRIBUTING.md instructions.

Fixes #139.

Change-Id: I517c3e6a8aefae7e08e49092e5fe350995ef1898

googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,blowmage/gcloud-golang,michael-grosshans/google-cloud-go,roth1002/gcloud-golang,hsinhoyeh/gcloud-golang,ptomasroos/gcloud-golang,googleapis/google-cloud-go,ptomasroos/gcloud-golang,googleapis/google-cloud-go,blowmage/gcloud-golang,googleapis/google-cloud-go,Maksadbek/gcloud-golang,michael-grosshans/google-cloud-go,roth1002/gcloud-golang,Maksadbek/gcloud-golang