Fixed #Q14 nested array flattening

Author: ayush013

challenges/collections-challenges.md

@@ -344,7 +344,7 @@ function flat(arr){
     const flatArr = [];
     arr.forEach((value) => {
         if(Array.isArray(value)){
-            flat(value);
+            flatArr.push(...flat(value));
         }
         else{
             flatArr.push(value);