docs: fix MultiAction.add() docstring return value

Author: mmonfared

appium/webdriver/common/multi_action.py

@@ -59,7 +59,7 @@ def add(self, *touch_actions: 'TouchAction') -> None:
             | MultiAction(driver).add(a1, a2)
 
         Returns:
-            `MultiAction`: Self instance
+            `MultiAction`: None
         """
         for touch_action in touch_actions:
             if self._touch_actions is None: