Fix repository URL in CONTRIBUTING

Change-Id: Ib8fffa4f7dcc2b5e6c3ebe0fcff14ac64a7b8683
Reviewed-on: http://review.couchbase.org/24742
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>