fix broken links in local docs; fixes #42

- Note:

With these changes, compiling docs for the web with the magic incantation:

  scribble --html
           +m
           --redirect-main http://docs.racket-lang.org
           --dest .
           parsack.scrbl

now produces the warning:

  "some cross references may be broken due to undefined tags"

Although all the links in the resulting html appear correct.