probes_to_wiki.rb: fix comment and reduce unnecessary array

* tool/probes_to_wiki.rb: fix usage comment.  use Enumerable#grep
  which yields each elements to reduce unnecessary array.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41912 b2dd03c8-39d4-4d8f-98ff-823fe69b080e