Update default generator example.
Update default generator example.

Add some missing elements:
require statement
`Generator` namespace

NeimadTL/ruby,NeimadTL/ruby,Insti/xruby,Insti/xruby,NeimadTL/ruby,Insti/exercism-ruby,Insti/exercism-ruby,exercism/xruby,exercism/xruby,Insti/exercism-ruby