Add port option in the example without a Dockerfile (#1008)

* Add port option in the example without a Dockerfile

Add '-p 80:80' to the example without a Dockerfile.
This expose the port 80 so that users can conect to the server through a web client.

httpd/content.md

@@ -29,7 +29,7 @@ $ docker run -dit --name my-running-app my-apache2
 If you don't want to include a `Dockerfile` in your project, it is sufficient to do the following:
 
 ```console
-$ docker run -dit --name my-apache-app -v "$PWD":/usr/local/apache2/htdocs/ httpd:2.4
+$ docker run -dit --name my-apache-app -p 8080:80 -v "$PWD":/usr/local/apache2/htdocs/ httpd:2.4
 ```
 
 ### Configuration