Update typo in docblock for Worker.php

*cough* I am going to be that guy... there is a typo in your docblock, bro. =).

Targets 4.1 branch as per comment in pull request #4664
Merge pull request #4678 from timgws/patch-2

Update typo in docblock for Worker.php
performUpdate should return false if no update is performed
Merge pull request #4773 from zupolgec/4.1

performUpdate should return false if no update is performed
Merge branch '4.1' into 4.2