fix an incorrect docker-gen parameter, other minor fixes to example

- Needed to fix the incorrect --only-published to -only-published 
- Think it's more likely to work for more people if change -only-published to -only-exposed
- added :ro to the sock, to match your other examples in your other project pages.

Author: RichLewis007

README.md

@@ -67,9 +67,9 @@ Fetch the template and start the docker-gen container with the shared volume:
 $ mkdir -p /tmp/templates && cd /tmp/templates
 $ curl -o nginx.tmpl https://raw.githubusercontent.com/jwilder/docker-gen/master/templates/nginx.tmpl
 $ docker run -d --name nginx-gen --volumes-from nginx \
-   -v /var/run/docker.sock:/tmp/docker.sock \
+   -v /var/run/docker.sock:/tmp/docker.sock:ro \
    -v /tmp/templates:/etc/docker-gen/templates \
-   -t jwilder/docker-gen:0.3.4 -notify-sighup nginx -watch --only-published /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
+   -t jwilder/docker-gen -notify-sighup nginx -watch -only-exposed /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
 ```
 
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