Add IsVictory logic into example code.

docs/documentation/tutorial.md

@@ -94,7 +94,30 @@ Let's also prevent players from being able to overwrite cells:
 ```js
 // Return true if `cells` is in a winning configuration.
 function IsVictory(cells) {
-  ...
+  const positions = [
+    [0, 1, 2],
+    [3, 4, 5],
+    [6, 7, 8],
+    [0, 3, 6],
+    [1, 4, 7],
+    [2, 5, 8],
+    [0, 4, 8],
+    [2, 4, 6],
+  ];
+
+  for (let pos of positions) {
+    const symbol = cells[pos[0]];
+    let winner = symbol;
+    for (let i of pos) {
+      if (cells[i] != symbol) {
+        winner = null;
+        break;
+      }
+    }
+    if (winner != null) return true;
+  }
+
+  return false;
 }
 
 // Return true if all `cells` are occupied.