doc: add single arg scenario for util.format

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

PR-URL: nodejs#12374
Fixes: nodejs#12362
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Alexey Orlenko <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: tarunbatra

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