fix indentation of example config

Otherwise I would get the following error when executing `bundle install`:

/usr/lib/ruby/1.9.1/psych.rb:203:in `parse': (/usr/share/foreman/config/settings.plugins.d/foreman-graphite.yaml): did not find expected key while parsing a block mapping at line 2 column 3 (Psych::S
yntaxError)

I added another line so the line number in the error message could differ.