Update 0055.跳跃游戏.md

python for循环版，添加了更新cover的条件

Author: roylx

problems/0055.跳跃游戏.md

@@ -126,8 +126,9 @@ class Solution:
         cover = 0
         if len(nums) == 1: return True
         for i in range(len(nums)):
-            cover = max(i + nums[i], cover)
-            if cover >= len(nums) - 1: return True
+            if i <= cover:
+                cover = max(i + nums[i], cover)
+                if cover >= len(nums) - 1: return True
         return False
 ```