fix a typo [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48123 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* signal.c (check_reserved_signal_): fix write count since r47991.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e