[TIDOC-1858] Update Tab.yml - iconIsMask wasn't spelled well

Check out: https://github.com/appcelerator/titanium_mobile/blob/master/iphone/Classes/TiUITabProxy.m#L592
As you can see, the name of the function is setIconIsMask. Currently in the doc we have setIconIsmask which is obviously wrong, because it doesnt exists. I've corrected the spelling mistake.