parse.y: fix a typo

* parse.y (regexp_contents): fix a typo.  pointed out by wanabe.
  [ruby-dev:48741] [Bug #10543]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48655 b2dd03c8-39d4-4d8f-98ff-823fe69b080e