docs: Update README.md to fix pip install llama cpp server (#1187)

Without the single quotes, when running the command, an error is printed saying no matching packages found on pypi. Adding the quotes fixes it

```bash
$ pip install llama-cpp-python[server]
zsh: no matches found: llama-cpp-python[server]
```

Co-authored-by: Andrei <abetlen@gmail.com>