doc: simplify process.memoryUsage() example code

Using util.inspect doesn't change the output in this case

PR-URL: https://github.com/nodejs/node/pull/9560


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>