string.c: escape a backslash [ci skip]

* string.c (rb_str_count): [DOC] add a backslash to escape a
  backslash itself.  [ruby-core:66321]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48465 b2dd03c8-39d4-4d8f-98ff-823fe69b080e