Update top_sort.md

Each node in the graph was being traversed irrespective of it being visited already in the previous traversals. Fixed that bug with the if condition in top_sort method's for loop.

Author: psnehas

graphs/top_sort.md

@@ -23,7 +23,7 @@ def top_sort(graph):
   return list(sorted_nodes)
 
 
-def dfs(graph, start_node, visited, sorted\_nodes):
+def dfs(graph, start_node, visited, sorted_nodes):
   visited.add(start_node)
   if start_node in graph:
       neighbors = graph[start_node]