assert: fix misformatted error message

Before: `Missing expected exception..`

Afer: `Missing expected exception.`

PR-URL: https://github.com/nodejs/node/pull/11254


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>