Fix typo in android_device (#764)

diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index 2c2375c..e793e21 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -439,7 +439,7 @@
   See the docs of `service_manager` and `base_service` for details.
 
   Attributes:
-    serial: A string that's the serial number of the Androi device.
+    serial: A string that's the serial number of the Android device.
     log_path: A string that is the path where all logs collected on this
       android device should be stored.
     log: A logger adapted from root logger with an added prefix specific