speech (docs): Alpha -> Beta in README
speech (docs): Alpha -> Beta in README

Fixes #2674

jgeewax/gcloud-node,GoogleCloudPlatform/gcloud-node,googleapis/google-cloud-node,googleapis/google-cloud-node,jgeewax/gcloud-node,GoogleCloudPlatform/gcloud-node,googleapis/google-cloud-node,GoogleCloudPlatform/gcloud-node,googleapis/google-cloud-node