(docs) Fix header levels of fact-contents endpoint
(docs) Fix header levels of fact-contents endpoint

The header levels for the fact-contents endpoint documentation didn't match other endpoint docs,
this corrects the levels so the style is normalized and the TOC looks correct.
rbrw/puppetdb,puppetlabs/puppetdb,kbarber/puppetdb,mullr/puppetdb,kbarber/puppetdb,senior/puppetdb,kbarber/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,senior/puppetdb,rbrw/puppetdb,senior/puppetdb,mullr/puppetdb,mullr/puppetdb,puppetlabs/puppetdb,rbrw/puppetdb,waynr/puppetdb,puppetlabs/puppetdb,rbrw/puppetdb,waynr/puppetdb,waynr/puppetdb,mullr/puppetdb,senior/puppetdb,kbarber/puppetdb,waynr/puppetdb,mullr/puppetdb,rbrw/puppetdb