Use string interpolation por concatting

Prevents the following:

vagrant@precise64 /web/services/rosi_catalog_service (master*) $ rake fdoc:to_html
/home/vagrant/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/bundler/gems/fdoc-830e0b954c5a/lib/fdoc/presenters/endpoint_presenter.rb:183:in `+': no implicit conversion of Fixnum into String (TypeError)
Merge pull request #2 from vemv/patch-2

Use string interpolation for concatting