README.EXT: fix typo

pointed out by at:
https://github.com/ruby/ruby/commit/c262acb4e023b6cd4e06b993508ab59a5f61607c#commitcomment-8748351

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48626 b2dd03c8-39d4-4d8f-98ff-823fe69b080e