Update 3d_gizmos.rst

As mentioned here https://github.com/godotengine/godot-docs/issues/6716
add/remove_node3d_gizmo_plugin is the wrong method name. But i'm not sure if it intentionally changed to add/remove_node_3d_gizmo_plugin.
Here are the adjustments anway in case it is intentional and going to stay that way.

tutorials/plugins/editor/3d_gizmos.rst

@@ -49,11 +49,11 @@ This would be a basic setup:
 
 
     func _enter_tree():
-        add_node3d_gizmo_plugin(gizmo_plugin)
+        add_node_3d_gizmo_plugin(gizmo_plugin)
 
 
     func _exit_tree():
-        remove_node3d_gizmo_plugin(gizmo_plugin)
+        remove_node_3d_gizmo_plugin(gizmo_plugin)
 
 
 For simple gizmos, inheriting :ref:`EditorNode3DGizmoPlugin <class_EditorNode3DGizmoPlugin>`