doc: unify quotes in an assert.md code example

PR-URL: https://github.com/nodejs/node/pull/12505


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>