update Migrations.sol template code

src/docs/truffle/getting-started/running-migrations.md

@@ -78,7 +78,8 @@ Truffle requires you to have a Migrations contract in order to use the Migration
 Filename: `contracts/Migrations.sol`
 
 ```solidity
-pragma solidity ^0.4.8;
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
 
 contract Migrations {
   address public owner;
@@ -87,19 +88,20 @@ contract Migrations {
   uint public last_completed_migration;
 
   modifier restricted() {
-    if (msg.sender == owner) _;
+    require(msg.sender == owner);
+    _;
   }
 
-  function Migrations() {
+  constructor() {
     owner = msg.sender;
   }
 
   // A function with the signature `setCompleted(uint)` is required.
-  function setCompleted(uint completed) restricted {
+  function setCompleted(uint completed) public restricted {
     last_completed_migration = completed;
   }
 
-  function upgrade(address new_address) restricted {
+  function upgrade(address new_address) public restricted {
     Migrations upgraded = Migrations(new_address);
     upgraded.setCompleted(last_completed_migration);
   }