Wrong type annotation for ndb.get_multi_async() and ndb.get_multi()

[vthorsteinsson]

In reference to the following two function declarations:

https://github.com/python/typeshed/blob/fa70187245751c6fd4d057534af7f9ae3e74a25f/stubs/google-cloud-ndb/google/cloud/ndb/model.pyi#L419

https://github.com/python/typeshed/blob/fa70187245751c6fd4d057534af7f9ae3e74a25f/stubs/google-cloud-ndb/google/cloud/ndb/model.pyi#L437

The type of the keys parameter should be Sequence[key_module.Key], not Sequence[type[key_module.Key]].