tunnel: fix RFC number in comment for INET_ECN_decapsulate()

The quoted text and figure are from RFC 6040 ("Tunnelling of Explicit
Congestion Notification").
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_ecn.h

@@ -150,7 +150,7 @@ static inline int INET_ECN_set_ce(struct sk_buff *skb)
 }
 
 /*
- * RFC 6080 4.2
+ * RFC 6040 4.2
  *  To decapsulate the inner header at the tunnel egress, a compliant
  *  tunnel egress MUST set the outgoing ECN field to the codepoint at the
  *  intersection of the appropriate arriving inner header (row) and outer