Add a link to the configuration file section from the cookbook.

I was trying to figure out how to configure pip to install wheel file.

The documentation prompted me to add an option to my `pip.ini` file but I had to search again where to put this file (and, as it turns out, the file is called `pip.conf` on my system).

This pull request changes the wording to be platform-neutral and adds a link to the right section so that the info is just one click away.