Fix website source link in CONTRIBUTING.md
Fix website source link in CONTRIBUTING.md

`docs/src/site` became `docs/src/hugo` in commit 787d5145.

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