[3.x] Add example for NodePath to grandparent

(cherry picked from commit b3ebc80feb)

doc/classes/NodePath.xml

@@ -15,6 +15,7 @@
 		@"." # The current node.
 		@".." # The parent node.
 		@"../C" # A sibling node C.
+		@"../.." # The grandparent node.
 		# A leading slash means it is absolute from the SceneTree.
 		@"/root" # Equivalent to get_tree().get_root().
 		@"/root/Main" # If your main scene's root node were named "Main".