contributing-doc: give example commit message with "CLA: trivial"

contributing-doc: give example commit message with "CLA: trivial"

The text "CLA: trivial" should go at the bottom of the commit message.
Also, update the force-push command to include the repository and
branch, which can avoid unexpected force-push results.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22775)