input: Write a function to calculate the average of a list of dictionaries where each dictionary contains a 'value' key.
  assert calculate_average([{'value': 10}, {'value': 20}, {'value': 30}]) == 20
  assert calculate_average([{'value': 5}, {'value': 15}, {'value': 25}, {'value': 35}]) == 20
  assert calculate_average([{'value': 100}, {'value': 200}, {'value': 300}, {'value': 400}, {'value': 500}]) == 300
output: def calculate_average(dict_list):
      total = sum(d['value'] for d in dict_list)
      return total / len(dict_list)