Update 0015.三数之和.md

添加了Python的双指针法

problems/0015.三数之和.md

@@ -218,7 +218,8 @@ class Solution {
 ```
 
 Python：
-```class Solution:
+```Python
+class Solution:
     def threeSum(self, nums):
         ans = []
         n = len(nums)