Fix doc comment of client.watch() (#164)

* Fix doc comment of client.watch()

The exception raised when the timeout expires is etcd.EtcdWatchTimedOut

Author: Gigi Sayfan

src/etcd/client.py

@@ -754,9 +754,9 @@ def watch(self, key, index=None, timeout=None, recursive=None):
             client.EtcdResult
 
         Raises:
-            KeyValue:  If the key doesn't exists.
+            KeyValue:  If the key doesn't exist.
 
-            urllib3.exceptions.TimeoutError: If timeout is reached.
+            etcd.EtcdWatchTimedOut: If timeout is reached.
 
         >>> print client.watch('/key').value
         'value'