strscan.c: fix typo

* ext/strscan/strscan.c (inspect1): fix typo, rb_str_cat() instead
  of rb_str_cat2().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e