update FAQ partials answer to point to sinatra-recipes project

there are 2 entries in sinatra-recipes about partials, i think the FAQ should point to them instead of documenting some of that content on its own. it will be easier to keep comprehensive and up to date if the content is in a single place.

i have a WIP article that i'm finishing up on Rails-style partials, available for review here: https://github.com/sinatra/sinatra-recipes/pull/69