doc: remove superfluous sample assert code

Remove superfluous sample code. Since `assert()` is documented as an
alias of `assert.ok()` and nothing more, the sample code for
`assert.ok()` is sufficient.

PR-URL: https://github.com/nodejs/node/pull/11933


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>