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>

doc/api/util.md

@@ -135,6 +135,13 @@ Each argument is converted to a string using `util.inspect()`.
 util.format(1, 2, 3); // '1 2 3'
 ```
 
+If only one argument is passed to `util.format()`, it is returned as it is
+without any formatting.
+
+```js
+util.format('%% %s'); // '%% %s'
+```
+
 ## util.inherits(constructor, superConstructor)
 <!-- YAML
 added: v0.3.0