QChar::Direction: more specific link to unicode standard

Giving the URL of unicode.org isn't much use; the standard is enormous
and the reader shall have trouble finding the specific table that
enumerates the direction types.  Found a more specific URL (that looks
like it should be stable) for this one link.

Change-Id: I8abcedf530baf692d7e0c7f146f3066df223951f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qchar.cpp

@@ -424,7 +424,7 @@ QT_BEGIN_NAMESPACE
     \enum QChar::Direction
 
     This enum type defines the Unicode direction attributes. See the
-    \l{http://www.unicode.org/}{Unicode Standard} for a description
+    \l{http://www.unicode.org/reports/tr9/tr9-35.html#Table_Bidirectional_Character_Types}{Unicode Standard} for a description
     of the values.
 
     In order to conform to C/C++ naming conventions "Dir" is prepended