doc: Sync classref with current source

doc/classes/EditorSpatialGizmoPlugin.xml

@@ -137,6 +137,12 @@
 				Override this method to provide the name that will appear in the gizmo visibility menu.
 			</description>
 		</method>
+		<method name="get_priority" qualifiers="virtual">
+			<return type="String">
+			</return>
+			<description>
+			</description>
+		</method>
 		<method name="has_gizmo" qualifiers="virtual">
 			<return type="bool">
 			</return>