Update 5.mergeSort.md

Author: JerryChii

5.mergeSort.md

@@ -89,9 +89,9 @@ def merge(left,right):
             result.append(left.pop(0))
         else:
             result.append(right.pop(0))
-    while left:
+    if left:
         result.extend(left)
-    while right:
+    if right:
         result.extend(right)
     return result
 ```