Update gossipsub-chat example docstring
//! window, run:
//!
//! ```sh
-//! cargo run --example chat
+//! cargo run --example gossipsub-chat
//! ```
//!
//! It will print the PeerId and the listening address, e.g. `Listening on
//! In the second terminal window, start a new instance of the example with:
//!
//! ```sh
-//! cargo run --example chat -- /ip4/127.0.0.1/tcp/24915
+//! cargo run --example gossipsub-chat -- /ip4/127.0.0.1/tcp/24915
//! ```
//!
//! The two nodes should then connect.
