vm_eval.c: fix `rb_eval_string_wrap` rdoc

* vm_eval.c (rb_eval_string_wrap): [DOC] Fix `rb_eval_string_wrap`
  documentation.  It is referencing `require` instead of `load`.
  The former does not have the optional argument.  [Fix GH-779]
  [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48745 b2dd03c8-39d4-4d8f-98ff-823fe69b080e