Fix outdated method name in code sample in 3D gizmo plugins (#9695)

Addresses "Unnamed Gizmo" error when following example code;
now overriding `_get_gizmo_name` function, not `get_name`.

tutorials/plugins/editor/3d_gizmos.rst

@@ -34,7 +34,7 @@ This would be a basic setup:
     extends EditorNode3DGizmoPlugin
 
 
-    func get_name():
+    func _get_gizmo_name():
         return "CustomNode"