doc: fix a typo in the assert.md

PR-URL: https://github.com/nodejs/node/pull/9598


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Roman Reiss <me@silverwind.io>