docs: align gateway test command with cli help

Use the canonical --interface flag in the README and MVP test guide so the
handoff command matches lanparty-gateway --help. Keep --iface documented as the
shorter accepted alias because the CLI and tests still support it.

Test Plan:
- ./target/release/lanparty-gateway --help
- cargo test -p lanparty-gateway accepts_iface_alias_for_gateway_interface
- git diff --check
- git diff --cached --check

Refs: MVP test handoff polish

README.md

@@ -173,7 +173,7 @@ cargo run -p lanparty-gateway -- \
   --server-name lanparty-relay.local \
   --relay-ca-cert relay-cert.der \
   --room ROOM1 \
-  --iface eth0
+  --interface eth0
 ```
 
 The gateway first opens the wired LAN interface as an AF_PACKET socket with