Inspector plugin tutorial: update PackedScene.instance() ref

I noticed that the tutorial still says `instance()` instead of `instantiate()`, so I changed it to match the 4.x method name.

(cherry picked from commit ef828fb837a65101c243b2aa8d0b0bed5a89a2fd)

tutorials/plugins/editor/inspector_plugins.rst

@@ -39,7 +39,7 @@ you should remove the instance you have added by calling
 ``remove_inspector_plugin()``.
 
 .. note:: Here, you are loading a script and not a packed scene. Therefore you
-          should use ``new()`` instead of ``instance()``.
+          should use ``new()`` instead of ``instantiate()``.
 
 .. tabs::
   .. code-tab:: gdscript GDScript