Update graph.md

tutorial/graph.md

@@ -26,7 +26,7 @@ There are also some functions for both classes like **has_edge(u, v)** that chec
 ### **DFS**:
 ```cpp
 #include <algoplus/graph>
-graph<int> g("directed);
+graph<int> g("directed");
 g.add_edge(1, 2);
 g.add_edge(4, 5);
 g.add_edge(5 , 6);