Update URL to the commit-msg hook in CONTRIBUTING doc

Change-Id: I617eb4768f38c62ee06cc5a93fea60ae1eef6050
Reviewed-on: http://review.couchbase.org/24744
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>