Update 815._Bus_Routes.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/815._Bus_Routes.md

@@ -56,12 +56,14 @@ class Solution(object):
             for bus in cur_buses:
                 if bus == T:
                     return step
+                
                 for route_idx in bus_in_routes[bus]:
                     if route_idx not in visited:
                         visited.add(route_idx)
                         for new_bus in routes[route_idx]:
                             if new_bus != bus:
                                 new_buses.append(new_bus)
+                        routes[route_idx] = [] # this line is optional, more efficient if added
             cur_buses = new_buses
             step += 1
         return -1