Add missing onSearchButtonPress option to native-stack-navigator.md

[kellycampbell]

This important option was missing from the documentation. I found it in the underlying library's documentation at

https://github.com/software-mansion/react-native-screens/blob/main/guides/GUIDE_FOR_LIBRARY_AUTHORS.md#screenstackheaderconfig

READ ME PLEASE

TL;DR: Make sure to add your changes to versioned docs

Thanks for opening a PR!

The docs cover several versions of react-navigation, and in some cases there are several files (for version 1, version 2 and etc.) that all describe a single page of the docs (eg. "Getting Started").

Please make sure that the edit you're making in docs/file-you-edited.md is also included in the file for the correct version, eg. /versioned_docs/version-3.x/file-you-edited.md for version 3. If such file doesn't exist, please create it. :+1: