Update 207._course_schedule.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/207._course_schedule.md

@@ -40,6 +40,7 @@ You may assume that there are no duplicate edges in the input prerequisites.
 
 > 思路 1
 ******- 时间复杂度: O(V+E)******- 空间复杂度: O(N)******
+
 [Topological sorting](https://www.geeksforgeeks.org/topological-sorting/) for Directed Acyclic Graph (DAG) is a linear ordering of vertices such that for every directed edge uv, vertex u comes before v in the ordering. Topological Sorting for a graph is not possible if the graph is not a DAG.
 
 pre-requisite问题，只需要判断最终的topological结果长度与courses数目是否相等即可