Fixing versioning comments for GA clients (#3411)

Author: garrettjonesgoogle

google-cloud-pubsub/README.md

@@ -216,9 +216,7 @@ Versioning
 
 This library follows [Semantic Versioning](http://semver.org/).
 
-It is currently in major version zero (``0.y.z``), which means that anything
-may change at any time and the public API should not be considered
-stable.
+It is currently in major version one (``1.y.z``), which means that the public API should be considered stable.
 
 Contributing
 ------------