Update flash-loans.md

updating imports to match with balancer-v2-monorepo
And add the return loan logic

docs/guides/arbitrageurs/flash-loans.md

@@ -9,18 +9,19 @@ Since the Vault holds all tokens for all pools, the consolidated token balances
 ```solidity
 pragma solidity ^0.7.0;
 
-import "@balancer-labs/v2-vault/contracts/interfaces/IVault.sol";
-import "@balancer-labs/v2-vault/contracts/interfaces/IFlashLoanRecipient.sol";
+/*
+* "@balancer-labs/v2-interfaces/contracts" is refering to  "balancer-v2-monorepo/pkg/interfaces/contracts"
+*/
+import {IVault} from "@balancer-labs/v2-interfaces/contracts/vault/IVault.sol";
+import {IFlashLoanRecipient} from "@balancer-labs/v2-interfaces/contracts/vault/IFlashLoanRecipient.sol";
+import {IERC20} from "@balancer-labs/v2-interfaces/contracts/solidity-utils/openzeppelin/IERC20.sol";
+
 
 contract FlashLoanRecipient is IFlashLoanRecipient {
-    IVault private constant vault = "0xBA12222222228d8Ba445958a75a0704d566BF2C8";
+    IVault private constant vault = IVault(0xBA12222222228d8Ba445958a75a0704d566BF2C8);
 
-    function makeFlashLoan(
-        IERC20[] memory tokens,
-        uint256[] memory amounts,
-        bytes memory userData
-    ) external {
-      vault.flashLoan(this, tokens, amounts, userData);
+    function makeFlashLoan(IERC20[] memory tokens, uint256[] memory amounts, bytes memory userData) external {
+        vault.flashLoan(this, tokens, amounts, userData);
     }
 
     function receiveFlashLoan(
@@ -29,8 +30,23 @@ contract FlashLoanRecipient is IFlashLoanRecipient {
         uint256[] memory feeAmounts,
         bytes memory userData
     ) external override {
-        require(msg.sender == vault);
-        ...
+        require(msg.sender == address(vault));
+
+        // This contract now has the funds requested.
+
+        // Your logic goes here...
+
+        /* 
+            At the end of your logic above, this contract owes
+            the flashloaned amounts + feeAmounts.
+            Therefore ensure your contract has enough to repay these amounts.
+        */
+
+        // Return loan
+        for (uint256 i = 0; i < tokens.length; i++) {
+            tokens[i].transfer(address(vault), amounts[i] + feeAmounts[i]);
+        }
     }
 }
+
 ```