Update spanner docs to show correct 'Batch.delete' usage.
Update spanner docs to show correct 'Batch.delete' usage.

Closes #3773.

jonparrott/gcloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python,calpeyser/google-cloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,tseaver/gcloud-python,tswast/google-cloud-python,tseaver/gcloud-python,dhermes/gcloud-python,tswast/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,calpeyser/google-cloud-python,dhermes/gcloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python