doc: fix typo in esm.md

Change `run time` to `runtime` for both correctness and consistency with
every other instance of the expression in the docs.

PR-URL: https://github.com/nodejs/node/pull/18142


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Jon Moss <me@jonathanmoss.me>