Minor improvements to contributors guide
Minor improvements to contributors guide

Fixed a couple markdown links and trimmed a few more words
to make things a bit less wordy.

ChristopherDavenport/http4s,ZizhengTai/http4s,rossabaker/http4s,ChristopherDavenport/http4s,http4s/http4s,ChristopherDavenport/http4s,ZizhengTai/http4s,aeons/http4s,reactormonk/http4s,reactormonk/http4s,reactormonk/http4s,aeons/http4s,ZizhengTai/http4s,aeons/http4s,rossabaker/http4s