ruby.h: no $

* include/ruby/ruby.h (ruby_safe_level_4): get rid of special
  character.  [ruby-dev:47512] [misc #8646]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42035 b2dd03c8-39d4-4d8f-98ff-823fe69b080e