tools: use latest rather than next for markdown linting plugin

eslint-plugin-markdown is being installed @next to get a bugfix for
https://github.com/eslint/eslint-plugin-markdown/issues/69 but that
bugfix is in 1.0.0. Go back to installing @latest rather than @next.

PR-URL: https://github.com/nodejs/node/pull/26345


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>