Remove / Update reference to HermitCore-rs toolchain

[n0toose]

This section mentions a repository that does not exist anymore: https://github.com/hermit-os/hermit-playground/?tab=readme-ov-file#hermitcore-rs-cross-toolchain

URL: https://github.com/hermit-os/hermit-toolchain