Fix a typo in docstring. (#516)

diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index 8058648..430ece9 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -860,7 +860,7 @@
         """.. deprecated:: 1.8
 
         Directly register with service manager instead:
-        `self.services.register('sl4a', sl4a_service)`
+        `self.services.register('sl4a', sl4a_service.Sl4aService)`
 
         Register sl4a_service directly instead.