Fix readme typos

A couple of the links in the readme are broken.  This fixes the
ones that I noticed.  Also fixes a typo in the release notes.

Change-Id: I244a187593dff11d554c2c8d86b0a531259a433f
(cherry picked from commit 36f942e8bfb69e59165e7a5ed03b16183add80b8)
Merge branch 'grizzly' of https://github.com/gwdg/puppetlabs-horizon into grizzly