Fix broken links in documentation.

Some documentation links currently point to pages that HP retired along with the rest of the SGI STL website. These links now point to the Boost mirror site.

using_concept_check.htm

@@ -43,7 +43,7 @@
   <tt>EqualityComparableConcept</tt> class. The class corresponds to the
   EqualityComparable requirements described in 20.1.1 of the C++ Standard,
   and to the <a href=
-  "http://www.sgi.com/tech/stl/EqualityComparable.html">EqualityComparable</a>
+  "http://www.boost.org/sgi/stl/EqualityComparable.html">EqualityComparable</a>
   concept documented in the SGI STL.</p>
   <pre>
 template &lt;class T&gt;