(maint) Fix link from 3.5 to 3.x reference guides

The 3.5 reference guide linked to `/puppet/3.0/reference`, but the
documentation is actually served from `/puppet/3/reference`.
Merge pull request #312 from Sharpie/fix-link-to-3.x-docs

(maint) Fix link from 3.5 to 3.x reference guides