Change enum RPChoiceAnswerStyle names

[Panosfunk]

inside `lib/src/model/answerformat/answer_format.dart the enum

/// Available types for [RPChoiceAnswerFormat]
enum RPChoiceAnswerStyle {
  /// The participant allowed to choose only a single option from the given set
  SingleChoice,

  /// The participant allowed only to choose multiple option from the given set
  MultipleChoice,
}

has misleading names.