Update bfs_exercise.md

Author: nikhilailani

algorithms/9_BreadthFirstSearch/bfs_exercise.md

@@ -9,12 +9,14 @@ Example: A, B, C, D, E, F are the nodes of the graph.
 For example, you are given following data: 
 
 ```
-data = {'A': {'B'},
+data = {
+        'A': {'B'},
         'B': {'A', 'C', 'D'},
         'C': {'B', 'E'},
         'D': {'B', 'E'},
         'E': {'C', 'D', 'F'},
-        'F': {'E'}}
+        'F': {'E'}
+}
 ```
 
 the resultant graph will be :-