COOK-2276 : cookbook apache2 documentation regarding listening ports doesn't match default attributes

The documentation here :

https://github.com/opscode-cookbooks/apache2/blob/master/README.md#general-settings

says :

    node['apache']['listen_ports'] - Ports that httpd should listen on. Default is an array of ports 80 and 443.

However the default attributes here :

https://github.com/opscode-cookbooks/apache2/blob/master/attributes/default.rb#L106

are set to :

    default['apache']['listen_ports'] = ["80"]

I've opted to update the documentation since I'm guessing that's the least impactful change.

Feel free to test and merge this pull request. Thanks.