Clarify a Shard sequence-off message

A warning message by the Shard stated that the "Heartbeat [is] off",
when it's actually the sequence that is off.

src/gateway/shard.rs

@@ -367,7 +367,7 @@ impl Shard {
         match *event {
             Ok(GatewayEvent::Dispatch(seq, ref event)) => {
                 if seq > self.seq + 1 {
-                    warn!("[Shard {:?}] Heartbeat off; them: {}, us: {}", self.shard_info, seq, self.seq);
+                    warn!("[Shard {:?}] Sequence off; them: {}, us: {}", self.shard_info, seq, self.seq);
                 }
 
                 match *event {