Update hello_host.rs

Added a reference in the example's comment header to advise updating the local Cargo.toml to enable the necessary features for seamless integration with the example logic.

bevy_matchbox/examples/hello_host.rs

@@ -1,4 +1,7 @@
 //! Runs both signaling with server/client topology and runs the host in the same process
+//!    Note: When building a signaling server ensure your cargo has been updated,
+//!    Enable the `signaling` feature in the `bevy_matchbox` dependency:
+//!    bevy_matchbox = { version = "0.n", features = ["signaling"] }
 //!
 //! Sends messages periodically to all connected clients.