Update collections-concepts.md

Author: sadanandpai

challenges/collections-concepts.md

@@ -113,7 +113,7 @@ arr.unshift(1, 2);              // [1, 2, 3, 4, 5, 6]
 const arr = [1, 2, 3];
 const otherArr = [4, 5, 6];
 arr.push(...otherArr);          // [1, 2, 3, 4, 5, 6]
-arr.unshift(...arr);            // [4, 5, 6, 1, 2, 3, 4, 5, 6]
+arr.unshift(...otherArr);            // [4, 5, 6, 1, 2, 3, 4, 5, 6]
 ```
 
 ###### Notes