Update default URLs for SBCL manual in contrib/slime-references.el

Node URLs in SBCL's manual have changed from

  http://www.sbcl.org/manual/NODE.html

to

  http://www.sbcl.org/manual#NODE

* contrib/slime-references.el (slime-sbcl-manual-root): changed
  http://www.sbcl.org/manual/ -> http://www.sbcl.org/manual
  (slime-lookup-reference-at-point): changed format string "%s%s.html"
  -> "%s#%s"