Fix README "4.5" bad reduce example

README.md

@@ -379,7 +379,7 @@ Other Style Guides
     const flat = {};
     [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
       const flatten = memo.concat(item);
-      flat[index] = memo.concat(item);
+      flat[index] = flatten;
     });
 
     // good