Fix Pub/Sub doc link
Fix Pub/Sub doc link

Addresses https://github.com/GoogleCloudPlatform/google-cloud-java/issues/2060.

rborer/google-cloud-java,shinfan/gcloud-java,vam-google/google-cloud-java,shinfan/gcloud-java,FirebasePrivate/google-cloud-java,shinfan/gcloud-java,rborer/google-cloud-java,vam-google/google-cloud-java,shinfan/gcloud-java,FirebasePrivate/google-cloud-java,rborer/google-cloud-java,vam-google/google-cloud-java,FirebasePrivate/google-cloud-java,mbrukman/gcloud-java,mbrukman/gcloud-java,rborer/google-cloud-java,mbrukman/gcloud-java,shinfan/gcloud-java,vam-google/google-cloud-java,rborer/google-cloud-java,mbrukman/gcloud-java,FirebasePrivate/google-cloud-java,FirebasePrivate/google-cloud-java,vam-google/google-cloud-java,vam-google/google-cloud-java