Fix a typo

It fixes an error for rabbit-slide without arguments:

    % rabbit-slide
    lib/rabbit/command/rabbit-slide.rb:232:in `default_command': undefined method `file' for File:Class (NoMethodError)
            from lib/rabbit/command/rabbit-slide.rb:65:in `parse_command_line_arguments'
            from lib/rabbit/command/rabbit-slide.rb:46:in `run'
            from lib/rabbit/command/rabbit-slide.rb:33:in `run'
            from bin/rabbit-slide:22:in `<main>'