acloud: local image slight changes

Correct acloud --help message and highlight optional info.

Bug:  120577726
Test: make acloud && acloud --help
Change-Id: Ieef7c9d6a690312223d622ba105f0ab1e3f7df6c
diff --git a/public/acloud_main.py b/public/acloud_main.py
index 30e18be..264560b 100644
--- a/public/acloud_main.py
+++ b/public/acloud_main.py
@@ -31,7 +31,9 @@
  - Create instances:
     1) To create a remote cuttlefish instance with the local built image.
        Example:
-       $ acloud create --local_image /tmp/image_dir
+       $ acloud create --local-image
+       Or specify built image dir:
+       $ acloud create --local-image /tmp/image_dir
     2) To create a local cuttlefish instance using the image which has been
        built out in your workspace.
        Example: