doc: "s/git apply/git am -3" in V8 guide

git apply does not preserve the original commit message. These updated
instructions offer a simpler flow for backporting.

PR-URL: https://github.com/nodejs/node/pull/10665


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>