string.c: update rdoc of rb_str_slice_bang()

* string.c (rb_str_slice_bang): [DOC] update return value against
  a fixnum, which has changed because of M17N.  [fix GH-631]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46376 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-08

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46377 b2dd03c8-39d4-4d8f-98ff-823fe69b080e