Update 04-flxgroup.html.md
Update 04-flxgroup.html.md

Added how to import the class. Beginners may have trouble to figure out that it's in a sub-package.
Gama11/flixel-docs,Doppp/flixel-docs,Doppp/flixel-docs,jcward/flixel-docs,Gama11/flixel-docs,flabbyfabby/flixel-docs,jcward/flixel-docs,flabbyfabby/flixel-docs,flabbyfabby/flixel-docs