Fix the link to the PuppetDB module detailed documentation

In a recent pull request I had removed the README_GETTING_STARTED.md file from
Github leaving a broken link in the curated docs.

This patch replaces the link with the forge page, which should be the
authoritative place for these docs.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #424 from kbarber/ticket/master/fix_link_to_puppetdb_module_readme

Fix the link to the PuppetDB module detailed documentation