Removing " a" (singular) in front of "ViewportContainers" (plural)

This is basically a test for me in order to learn how to properly submit PRs, while also doing something tiny bit useful.

My 4th attempt at doing this correctly. This time from separate branch to master :)

tutorials/scripting/change_scenes_manually.rst

@@ -130,7 +130,7 @@ a scene's data between scene changes (adding the scene to the root node).
 
         GetTree().GetRoot().AddChild(scene);
 
-Perhaps instead they wish to display multiple scenes at the same time using a
+Perhaps instead they wish to display multiple scenes at the same time using
 :ref:`ViewportContainers <class_ViewportContainer>`. This is optimal in
 cases where the intent is to render different content in different parts of the
 screen. Minimaps and split-screen multiplayer are good examples.