scripts: test_image: drop mention of {u,}mount_image.sh

This script has never cared about these files, so stop mentioning them.

BUG=b:187789324
TEST=CQ passes

Change-Id: Ief0c3257bef73e21c0817373a9ec41f04f1a0515
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4128781
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Sergey Frolov <sfrolov@google.com>
Commit-Queue: Sergey Frolov <sfrolov@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/test_image.py b/scripts/test_image.py
index 8ffbf9a..8f3a7e9 100644
--- a/scripts/test_image.py
+++ b/scripts/test_image.py
@@ -28,8 +28,7 @@
     parser.add_argument(
         "image",
         type="path",
-        help="Image directory (or file) with mount_image.sh and "
-        "umount_image.sh",
+        help="Image directory or file to test",
     )
 
     parser.add_argument(