fix links

docs/modules/ROOT/pages/jme3/features.adoc

@@ -275,7 +275,7 @@ a|OGG Vorbis music and sounds
 == GUI / HUD
 
 *  xref:core:ui/hud.adoc[Orthogonal (Billboard) node]
-*  xref:jme3/advanced/nifty_gui.adoc[Nifty GUI integration]
+*  xref:core:gui/nifty_gui.adoc[Nifty GUI integration]
 
 
 == Miscellaneous

docs/modules/tutorials/pages/concepts/faq.adoc

@@ -640,7 +640,7 @@ You may want to display buttons to let the player switch between the game, setti
 
 *Learn more:*
 
-* xref:ROOT:jme3/advanced/nifty_gui.adoc[Nifty GUI]
+* xref:core:gui/nifty_gui.adoc[Nifty GUI]
 
 *Code Sample:*