string.c: fix typo

* string.c (rb_str_ellipsize): [DOC] fix typo, "encoding" instead of
  "encoded" which is probably a slip of the auto-completion.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42395 b2dd03c8-39d4-4d8f-98ff-823fe69b080e