doc: rename stackStartFunction in assert.md

[assert.js](https://github.com/nodejs/node/blob/master/lib/assert.js)
uses `stackStartFn` everywhere instead of `stackStartFunction`.

This also increases consistency with `stackStartFn`
in the `AssertionError` options.

PR-URL: https://github.com/nodejs/node/pull/22077


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>