datastore: update broken doc examples
datastore: update broken doc examples

Change-Id: I366b9de61d578af088e442b319879c5d2cb90df0
Reviewed-on: https://code-review.googlesource.com/10190
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go