ui: Correct links in `tools/ui/README.md` [no ci] (#23139)
In `tools/ui/README.md`, update the relative links, now that the `README.md` file has been moved from `tools/server/webui/` to `tools/ui/`.
See https://github.com/ggml-org/llama.cpp/commit/59778f0196a82db32580bb649d5d839355d6d7bf.
