pubsub: fix package doc
pubsub: fix package doc

Fixes #695.

Change-Id: Ie1d9dde327e785a41aea9d3c67acafa1fdf0d10c
Reviewed-on: https://code-review.googlesource.com/14611
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

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