input: Write a function to evaluate the accuracy of a machine-learning model using the provided input and output instructions. The function should take the model, training data (X_train, y_train), and testing data (X_test, y_test) as inputs and return the accuracy score as a string.
  assert evaluate_accuracy(clf, X_train, y_train, X_test, y_test) == "Accuracy: 0.95"
output: def evaluate_accuracy(model, X_train, y_train, X_test, y_test):
      model.fit(X_train, y_train)
      accuracy = model.score(X_test, y_test)
      return f"Accuracy: {accuracy:.2f}"