Fix calling the path for calling PuppetDB API

Previous to this commit, the Puppet::Client#request method incorrectly
constructed the path to call the PuppetDB API.  This commit uses the
value returned from self.class.base_uri to construct it correctly.

Further, the REAME did not specify the need to call specify which
endpoint to call on the PuppetDB API.  This commit updates the README to
reflect the proper use of the PuppetDB::Client#request method