Missing custom Display implementation on error (#2902)

Right now the `derive_more` library just prints the value of
`notifications_protocol_index` (an integer) when displaying this error,
which is kind of crappy.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

src/libp2p/peers.rs

@@ -2129,6 +2129,7 @@ pub enum InboundError {
     Connection(collection::InboundError),
     /// Refused a notifications substream because we already have an existing substream of that
     /// protocol.
+    #[display(fmt = "Refused duplicate notifications substream")]
     DuplicateNotificationsSubstream {
         /// Notifications protocol the substream is about.
         notifications_protocol_index: usize,