consensus/ethash: fix overdue link (#15786)

consensus/ethash/consensus.go

@@ -381,7 +381,7 @@ func calcDifficultyByzantium(time uint64, parent *types.Header) *big.Int {
 // the difficulty that a new block should have when created at time given the
 // parent block's time and difficulty. The calculation uses the Homestead rules.
 func calcDifficultyHomestead(time uint64, parent *types.Header) *big.Int {
-	// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.mediawiki
+	// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.md
 	// algorithm:
 	// diff = (parent_diff +
 	//         (parent_diff / 2048 * max(1 - (block_timestamp - parent_timestamp) // 10, -99))