switch usage to show options for enable script

Author: binhex

README.md

@@ -23,7 +23,7 @@ docker run -d \
     -v /etc/localtime:/etc/localtime:ro \
     -e WEBPAGE_TITLE=<name shown in browser tab> \
     -e VNC_PASSWORD=<password for web ui> \
-    -e ENABLE_STARTUP_SCRIPTS=<install additional packages> \
+    -e ENABLE_STARTUP_SCRIPTS=<yes|no> \
     -e UMASK=<umask for created files> \
     -e PUID=<uid for user> \
     -e PGID=<gid for user> \