Adding missing self variable (donnemartin#158)

solutions/object_oriented_design/parking_lot/parking_lot.py

@@ -64,7 +64,7 @@ def __init__(self, num_levels):
         self.levels = []  # List of Levels
 
     def park_vehicle(self, vehicle):
-        for level in levels:
+        for level in self.levels:
             if level.park_vehicle(vehicle):
                 return True
         return False