fix xref

docs/modules/tutorials/pages/beginner/hello_terrain.adoc

@@ -419,4 +419,4 @@ You have learned how to create terrains that are more efficient than loading one
 *See also:*
 
 *  xref:ROOT:jme3/advanced/terrain_collision.adoc[Terrain Collision]
-*  xref:ROOT:jme3/advanced/terrain.html.adoc[Advanced Terrain]
+*  xref:ROOT:jme3/advanced/terrain.adoc[Advanced Terrain]