doc: correcting misspelling

Changing `stringp` to `string`.

PR-URL: https://github.com/nodejs/node/pull/7797


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>