webtop: update app section (#116)

Removed reference to opensync with broken link.
Replaced with DAVx5

webtop.rst

@@ -254,9 +254,8 @@ synchronizable resources, as in the case of mobile devices (i.e. Android or iOS
 Google Android
 --------------
 
-A good, free, Android third-party client is `Opensync <https://deependhulla.com/android-apps/opensync-app>`_.
+A good Android third-party client is `DAVx5 <https://www.davx5.com/>`_.
 
-- install the suggested app from the store
 - add a new account clicking on :guilabel:`+` key and select :guilabel:`Login with URL and username` method
 - insert the ``URL`` (``https://<server_name>``), full username (i.e. *[email protected]*) and password
 - click on the new profile and select the resources you want to synchronize