puma/launcher: Fixed typo

[perlun]

I noted this while trying to apply suggestions from #1363.

The previous example causes exceptions when you try to run it:

FATAL [2017-10-05 11:11:17.535] UxFactory::Server: A fatal error occurred. Details below.
FATAL [2017-10-05 11:11:17.535] UxFactory::Server: no implicit conversion of Puma::Events into Array (TypeError)
        /Users/plundberg/.rvm/gems/ruby-2.4.2/gems/puma-3.10.0/lib/puma/launcher.rb:370:in `generate_restart_data'
        /Users/plundberg/.rvm/gems/ruby-2.4.2/gems/puma-3.10.0/lib/puma/launcher.rb:64:in `initialize'
        /Volumes/extra/git/ecraft/ecraft.uxfactory.server-8.x/src/server/lib/server/uxfactory_server.rb:32:in `new'
        /Volumes/extra/git/ecraft/ecraft.uxfactory.server-8.x/src/server/lib/server/uxfactory_server.rb:32:in `start_server'
        /Volumes/extra/git/ecraft/ecraft.uxfactory.server-8.x/src/server/lib/server/uxfactory_server.rb:15:in `start'
        /Volumes/extra/git/ecraft/ecraft.uxfactory.server-8.x/src/uxfactory/server.rb:69:in `block in start!'