Minor updates applied to [URL] reference proposal

The following minor changes were made to the text, primarily in response 
to comments.  As a reminder, this effort is focused on producing a 
recommendation for a replacement for the informative text contained in 
the [URL] reference.  Any other comments should be directed to W3C 
Management at this point.

* s/Duerst/Dürst/
   In response to:
   http://lists.w3.org/Archives/Public/www-tag/2014Oct/0010.html

* s/mapped directly/straightforwardly mapped/
   In response to:
   http://lists.w3.org/Archives/Public/www-tag/2014Oct/0018.html

* s|http://url.spec.whatwg.org/|https://url.spec.whatwg.org/|
   In response to:
   http://lists.w3.org/Archives/Public/www-tag/2014Oct/0012.html

* s/relatives schemes/relative schemes/
   Not reported elsewhere, just happened to spot it.

You can see these changes applied at:

   http://dev.w3.org/html5/2014/10/url-ref.html

- Sam Ruby