Fix illogical code in tutorial.md (#8048)

The tutorial wants to throw a 'warning' and explains about 'key' of React's list, but it throws nothing since there is sensible key.
"key={move}" should be removed, and added after explaining about key.

tutorial/tutorial.md

@@ -414,7 +414,7 @@ const moves = history.map((step, move) => {
     'Move #' + move :
     'Game start';
   return (
-    <li key={move}>
+    <li>
       <a href="#" onClick={() => this.jumpTo(move)}>{desc}</a>
     </li>
   );