Bump Firestore to Beta (#4261)
Bump Firestore to Beta (#4261)

Correcting Firestore release level Beta, not Alpha
jonparrott/google-cloud-python,tseaver/google-cloud-python,dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python,jonparrott/google-cloud-python,dhermes/gcloud-python,googleapis/google-cloud-python,jonparrott/gcloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,jonparrott/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tseaver/gcloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python,dhermes/google-cloud-python,tseaver/gcloud-python,tswast/google-cloud-python