server: update README.md with llama-server --help output [no ci] (#8472)

The README.md had a stale information. In particular, the --ctx-size
"defaults to 512" confused me and I had to check the code to confirm
this was false. This the server is evolving rapidly, it's probably
better to keep the source of truth at a single place (in the source) and
generate the README.md based on that.

Did:

    make llama-server
    ./llama-server --help > t.txt
    vimdiff t.txt examples/server/README.md

I copied the content inside a backquote block. I would have preferred
proper text but it would require a fair amount of surgery to make the
current output compatible with markdown. A follow up could be to
automate this process with a script.

No functional change.