Fix for readme documentation link (#266)

The [previous link](http://www.haxeflixel.com/) is no longer the current domain of HaxeFlixel. So this is a quick fix for that

README.md

@@ -10,7 +10,7 @@ This is the main location for HaxeFlixel documentation. Pull requests from this
 
 ## Contents
 
-* The "main" documentation available on [haxeflixel.com/documentation](http://www.haxeflixel.com/documentation).
+* The "main" documentation available on [haxeflixel.com/documentation](https://haxeflixel.com/documentation).
 * The API documentation.
 
 ### Main documentation