doc: update contrib.html
doc: update contrib.html

Fix all the local links, as contrib.html is served as
/project/

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/5656105

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go