doc: use kbd element in readline doc prose

The kbd element has already been added in the keybindings table, but we
missed it in one line in the prose.

PR-URL: https://github.com/nodejs/node/pull/35737


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>