Acceptance Tests: remove unnecessary tag from documentation example

The ":avocado: enable" is not necessary and was removed in 9531d26,
so let's remove from the docs.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210203172357.1422425-4-crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

docs/devel/testing.rst

@@ -765,9 +765,6 @@ and hypothetical example follows:
 
 
   class MultipleMachines(Test):
-      """
-      :avocado: enable
-      """
       def test_multiple_machines(self):
           first_machine = self.get_vm()
           second_machine = self.get_vm()