doc/extension.rdoc: wording fix w.r.t. volatile vs RB_GC_GUARD
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56411 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
doc/extension.rdoc: wording fix w.r.t. volatile vs RB_GC_GUARD
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56411 b2dd03c8-39d4-4d8f-98ff-823fe69b080e