Fixed typo in hyperlink.

[SVN r37519]

index.htm

@@ -222,7 +222,7 @@
 
                           <li>
                             <span class="library"><a href=
-                            "doc/hmtl/date_time.html">Date_time library</a>:</span>
+                            "doc/html/date_time.html">Date_time library</a>:</span>
                             <ul>
                               <li>Support for new US/Canada timezone rules and
                                   other bug fixes.  See