A newbie question about setting value for {{var}}

I study RailSpace code and the line code in the book

@error_messages = ActiveRecord::Errors.default_error_messages

has been deprecated. Instead the following is recommended

@error_messages = I18n.translate(‘activerecord.errors.messages’)

But the line
correct_error_message = sprintf(@error_messages[:too_short],

does not assign the value min_length to {{count}} part of
@error_messages[:too_short] “is too short (minimum is {{count}}
characters)” .

I do not know how to set the min_length value to {{count}} in