Rephrase the create-fixed-helper usage msg

Make it reflect the current state

snf-image-host/snf-image-create-fixed-helper.in

@@ -30,9 +30,9 @@ usage() {
 
 Usage: $0 [options]
 
-This script runs a debootstrap and creates a small Debian image populated with
-the snf-image-helper package. This needs to be done before the first use of
-ganeti's \`snf-image' guest OS type.
+This script runs multistrap and creates a small Debian image populated with
+the snf-image-helper package. This image is needed for ganeti's \`snf-image'
+guest OS type to work.
 
 OPTIONS:
     -d DIRECTORY