raft: Fix comment on sendHeartbeat

raft/raft.go

@@ -529,7 +529,7 @@ func (r *raft) maybeSendAppend(to uint64, sendIfEmpty bool) bool {
 	return true
 }
 
-// sendHeartbeat sends an empty MsgApp
+// sendHeartbeat sends a heartbeat RPC to the given peer.
 func (r *raft) sendHeartbeat(to uint64, ctx []byte) {
 	// Attach the commit as min(to.matched, r.committed).
 	// When the leader sends out heartbeat message,