AUM: usb unplug: additional hint for user

Additional hint for user stating when he need to remove the flash drive.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

test-cases/apertis-update-manager-usb-unplug.yaml

@@ -30,12 +30,12 @@ run:
     - $ sudo journalctl -ef --unit apertis-update-manager &
     - "Plug the USB flash drive into the device"
     - "The update starts automatically"
-    - "Wait while the upgrade starts"
+    - "Wait while the upgrade starts and remove the USB flash before it is completed, as described below"
     - |
         >Dec 19 22:31:46 apertis apertis-update-[390]: mount added : /media/test
         Dec 19 22:31:46 apertis apertis-update-[390]: Ostree static delta starting
         Dec 19 22:31:46 apertis apertis-update-[390]: Cannot check the ID in black list: No such file or directory
-    - "Wait 10-20 seconds (depending on your HW) and remove the USB flash drive"
+    - "Wait 5-20 seconds (depending on your HW) and remove the USB flash drive"
     - "Update should be interrupted with error messages similar to"
     - |
         >[   59.542717] scsi 1:0:0:0: rejecting I/O to offline device