datastore: remove beta note from doc comment
datastore: remove beta note from doc comment

The package is stable.

Change-Id: Ib1f9314dfe78687bcee790f1d110d60fc4e3258e
Reviewed-on: https://code-review.googlesource.com/22951
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go