Fix 'Output is not a module'

[#46491831]

I missed that Rex::Ui::Text::Output was a class and not a module, so
starting up prosvc fell over when it loaded
rex/ui/text/output/buffer/stdout, which also would screw up
msf/ui/console/command_dispatcher/core.rb where I original added
Rex::Ui::Text::Output::Buffer::Stdout.