Missing scons command in GDExtension C++ example docs, also potentially outdated command when git cloning

**Your Godot version:** 4.2

**Issue description:** Missing command (and possibly an outdated one also) in docs

**URL to the documentation page:** https://docs.godotengine.org/en/stable/tutorials/scripting/gdextension/gdextension_cpp_example.html

The docs say:

"To generate and compile the bindings, use this command (replacing with windows, linux or macos depending on your OS):

The build process automatically detects the number of CPU threads [...]"

So there is a missing command in between those lines. I believe the missing command is `scons platform=` but this is my first time doing this so idk. There is also a call to scons later on, so maybe this one is redundant.

Also, I noticed that when cloning the repo it forces the 4.1 branch, but I think it should clone the 4.2 (?)

Hope this helps