doc: remove double line break

This is the only double line break in the file and for
consistency one is removed.

PR-URL: https://github.com/nodejs/node/pull/17581


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>