Fix folder name for external pillars in docs

Trying to get an external pillar to work, I realised that the folder containing the built-in external pillars are in ``salt/pillar``, the docs explain, however, that it should be ``pillars``. Is the difference between the names intentional? Or is this a typo in the documentation? 

In case it's the latter, I attached a PR with appropriate changes in the docs and the man page.