Fix README to install spanner instead of datastore. (#7301)
Fix README to install spanner instead of datastore. (#7301)


GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,tseaver/google-cloud-python