raft: fix tiny mistake of message type

raft/raft_test.go

@@ -873,8 +873,8 @@ func TestHandleHeartbeat(t *testing.T) {
 		m       pb.Message
 		wCommit uint64
 	}{
-		{pb.Message{From: 2, To: 1, Type: pb.MsgApp, Term: 2, Commit: commit + 1}, commit + 1},
-		{pb.Message{From: 2, To: 1, Type: pb.MsgApp, Term: 2, Commit: commit - 1}, commit}, // do not decrease commit
+		{pb.Message{From: 2, To: 1, Type: pb.MsgHeartbeat, Term: 2, Commit: commit + 1}, commit + 1},
+		{pb.Message{From: 2, To: 1, Type: pb.MsgHeartbeat, Term: 2, Commit: commit - 1}, commit}, // do not decrease commit
 	}
 
 	for i, tt := range tests {