docs: add link to multiabi page

Author: ChmielewskiKamil

Diff for: program-analysis/echidna/Exercise-X.md

@@ -406,6 +406,8 @@ Add a `Player(address player)` event to your contract and emit it in your proper
 
 The player's address is `0x0000...3000`. Echidna is making calls from multiple accounts. It was able to increase the allowance of an address `0x000...1000` and make a call to `transferFrom(player, 0x000...1000, amount)`. This is expected!
 
+[This happens because of the `multi-abi`](https://github.com/crytic/building-secure-contracts/blob/master/program-analysis/echidna/using-multi-abi.md). 
+
 The `lockTokens` modifier does not prevent others from making transfers. Only the player is constrained.
 
 ```