input: Write a python function to check if a task exists in the database by its description.
  assert task_exists('Go shopping')==True
  assert task_exists('Write unit tests')==False
  assert task_exists('Complete project')==True
output: def task_exists(description):
      tasks = [
          {'id': 1, 'description': 'Go shopping'},
          {'id': 2, 'description': 'Complete project'},
          {'id': 3, 'description': 'Read a book'}
      ]
      
      for task in tasks:
          if task['description'] == description:
              return True
      return False