Update 'uses' links in waitForExist.js
Update 'uses' links in waitForExist.js

The links on http://webdriver.io/api/utility/waitForExist.html are stale. Have updated the comments which I believe were used to autogenerate the documentation.
sebastiendavid/webdriverio,sebastiendavid/webdriverio,monolithed/webdriverio,monolithed/webdriverio