pull: Fix the usage and help text

Change-Id: I6ed6dfc1267f6bb05b3e5fc9fe8310b0d5329066
diff --git a/mdt/files.py b/mdt/files.py
index d9803e2..5592c54 100644
--- a/mdt/files.py
+++ b/mdt/files.py
@@ -149,7 +149,7 @@
 
 
 class PullCommand(command.NetworkCommand):
-    '''Usage: mdt pull [<filename...>]
+    '''Usage: mdt pull <filename...> <local-destination-path>
 
 Variables used:
     preferred-device    - set this to your preferred device name to connect
@@ -168,7 +168,7 @@
 
     def preConnectRun(self, args):
         if len(args) < 3:
-            print("Usage: mdt pull [<filename...>]")
+            print("Usage: mdt pull <filename...> <local-destination-path>")
             return False
 
         return True