Update documentation for custom AudioStreams

**Your Godot version:** 4.3

**Issue description:** The page for making custom AudioStreams seems to be outdated

**URL to the documentation page:** https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/custom_audiostreams.html

Trying to follow along and then building tells me that core/reference.h, core.resource.h, and core/print_string.h all don't exist. Godot builds fine without my custom module. I copied and pasted the exact code from the guide, only changing the names of the files and classes.