pubsub: fix package name in go_mod_tidy_hack.go
This will be tagged pubsub/v1.0.1.
// the resultant binary.
// +build modhack
-package datastore
+package pubsub
// Necessary for safely adding multi-module repo. See: https://github.com/golang/go/wiki/Modules#is-it-possible-to-add-a-module-to-a-multi-module-repository
import _ "cloud.google.com/go"
