Fix Ripper::Lexer::State#[] for to_s

The instance variable is called `to_s`, not `event`.

Author: Earlopain

ext/ripper/lib/ripper/lexer.rb

@@ -68,7 +68,7 @@ def [](index)
         when 0, :to_int
           @to_int
         when 1, :to_s
-          @event
+          @to_s
         else
           nil
         end