chore(iroh-relay): Fix README.md instuctions to enable server feature (#3239)

## Description

add feature "server" while running iroh-relay via cargo run

## Breaking Changes

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including how to migrate older code. -->

## Notes & open questions

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## Change checklist
<!-- Remove any that are not relevant. -->
- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.
- [x] List all breaking changes in the above "Breaking Changes" section.
- [x] Open an issue or PR on any number0 repos that are affected by this
breaking change. Give guidance on how the updates should be handled or
do the actual updates themselves. The major ones are:
    - [x] [`quic-rpc`](https://github.com/n0-computer/quic-rpc)
    - [x] [`iroh-gossip`](https://github.com/n0-computer/iroh-gossip)
    - [x] [`iroh-blobs`](https://github.com/n0-computer/iroh-blobs)
    - [x] [`dumbpipe`](https://github.com/n0-computer/dumbpipe)
    - [x] [`sendme`](https://github.com/n0-computer/sendme)

Author: arcinston

Diff for: iroh-relay/README.md

@@ -58,7 +58,7 @@ manual_key_path = "/path/to/certs/cert.key.pem"
 ```
 
 Then run the server with the `--dev` flag, like you would when normally testing locally:
-`cargo run --bin iroh-relay -- --config-path=/path/to/config.toml --dev`
+`cargo run --features="server" --bin iroh-relay -- --config-path=/path/to/config.toml --dev`
 
 The relay server will run over http on port 3340, as it does using the `--dev` flag, but it will also run a QUIC server on port 7824.