Fix `alpha` documentation
Fix `alpha` documentation

The documentation on http://www.openfl.org/developer/documentation/api/source/content/flash/display/DisplayObject.html for `var alpha` is that of `accessibilityProperties`
Gama11/openfl,openfl/openfl,NenadBojkovski/openfl,openfl/openfl,Gama11/openfl,NenadBojkovski/openfl,openfl/openfl,Gama11/openfl,Gama11/openfl