Fix typo in io19.rb

This commit fixes a typo which was causing rdoc to fail


$ RBXOPT=-X19 bin/rbx -S gem install rdoc
Fetching: rdoc-4.0.0.gem (100%)
Depending on your version of ruby, you may need to install ruby rdoc/ri data:

<= 1.8.6 : unsupported
 = 1.8.7 : gem install rdoc-data; rdoc-data --install
 = 1.9.1 : gem install rdoc-data; rdoc-data --install
>= 1.9.2 : nothing to do! Yay!
Successfully installed rdoc-4.0.0
1 gem installed
Installing ri documentation for rdoc-4.0.0...
Before reporting this, could you check that the file you're documenting
has proper syntax:

  /home/adrian/dev/adzankich/rubinius/bin/rbx -c Rakefile

RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.

The internal error was:

        (NameError) undefined local variable or method `evailable' on an instance of IO::EachReader.

ERROR:  While generating documentation for rdoc-4.0.0
... MESSAGE:   undefined local variable or method `evailable' on an instance of IO::EachReader.
... RDOC args: --ri --op /home/adrian/dev/adzankich/rubinius/gems/1.9/doc/rdoc-4.0.0/ri --main README.rdoc lib CVE-2013-0256.rdoc DEVELOPERS.rdoc History.rdoc LEGAL.rdoc LICENSE.rdoc Manifest.txt README.rdoc RI.rdoc TODO.rdoc bin/rdoc Rakefile --title rdoc-4.0.0 Documentation --quiet