Add direct link to validator registration contract

specs/core/0_beacon-chain.md

@@ -1378,7 +1378,7 @@ When sufficiently many full deposits have been made the deposit contract emits t
 
 ### Vyper code
 
-The source for the Vyper contract lives in a separate repository at [https://github.com/ethereum/deposit_contract](https://github.com/ethereum/deposit_contract).
+The source for the Vyper contract lives in a [separate repository](https://github.com/ethereum/deposit_contract) at [https://github.com/ethereum/deposit_contract/blob/master/deposit_contract/contracts/validator_registration.v.py](https://github.com/ethereum/deposit_contract/blob/master/deposit_contract/contracts/validator_registration.v.py).
 
 Note: to save ~10x on gas this contract uses a somewhat unintuitive progressive Merkle root calculation algo that requires only O(log(n)) storage. See https://github.com/ethereum/research/blob/master/beacon_chain_impl/progressive_merkle_tree.py for an implementation of the same algo in python tested for correctness.