Update 3.5.4_puppet_reports.md

Change the foreman-report_v2.rb.erb url to the one at 2.1_stable branch.
Puppet-foreman's master has no longer the file foreman-report_v2.rb.erb.

See more here:
https://groups.google.com/forum/#!topic/foreman-dev/XA2tXbNlPq8