doc: update code language flag for internal doc

Use `bash` instead of `shell` for code language flag in
doc/guides/maintaining-ngtcp2-nghttp3.md to conform with our other docs
and upcoming lint requirements.

PR-URL: https://github.com/nodejs/node/pull/33852


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>