Datastore transaction snippets all use `Transaction()` instead of `datastore.Transaction()`

[jgeewax]

Looking at this page (in it's entirety): http://googlecloudplatform.github.io/gcloud-python/latest/datastore-transactions.html

Minor thing here, but would be more clear if we explicitly wrote datastore.Transaction.

Basically s/Transaction()/datastore.Transaction()/

[dhermes]

@jgeewax I had that same thought when I wrote #689.

Fixed in master:
http://googlecloudplatform.github.io/gcloud-python/master/datastore-transactions.html