Update URL to commit-msg hook in CONTRIBUTING doc

Change-Id: I65cebc87d1a8ed3185768d49ca57a73ee967ecd4
Reviewed-on: http://review.couchbase.org/24745
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>