input: Write a python function to find a user by name from a list of users.
  assert find_user(users, "Alice") == {"name": "Alice", "age": 21}
  assert find_user(users, "John") == {"name": "John", "age": 26}
  assert find_user(users, "Mark") == {"name": "Mark", "age": 32}
  assert find_user(users, "Bob") == None
output: def find_user(users, name):
      for user in users:
          if user['name'] == name:
              return user
      return None