fix: MultiAction.add() return

closes #956

Author: mmonfared

appium/webdriver/common/multi_action.py

@@ -45,7 +45,7 @@ def __init__(self, driver: 'WebDriver', element: Optional['WebElement'] = None)
         self._element = element
         self._touch_actions: List['TouchAction'] = []
 
-    def add(self, *touch_actions: 'TouchAction') -> None:
+    def add(self, *touch_actions: 'TouchAction') -> 'MultiAction':
         """Add TouchAction objects to the MultiAction, to be performed later.
 
         Args: