doc: add single arg scenario for util.format

Set the expected outcome of `util.format('%%')` to be `%%`
instead of `%`.

PR-URL: https://github.com/nodejs/node/pull/12374
Fixes: https://github.com/nodejs/node/issues/12362


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>