Fix RA recommended configuration

[spastorino]

https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc

it mentions stage0-sysroot, more discussion https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/empty.20build.2F.24HOST.20directory

Is not clear what's the right suggestion

cc @jyn514

[jyn514]

This config is no longer maintained in the guide itself. The version in the rust-lang/rust repo has been fixed.