Fix <iref> element

Author: jwakely

xml/issue4118.xml

@@ -55,7 +55,7 @@ which we know how to format?
 </p>
 
 <p>
-This is somewhat related to issue <iref num="953"/>, since it's unclear
+This is somewhat related to issue <iref ref="953"/>, since it's unclear
 which operations "a class emulating an arithmetic type" needs to support.
 </p>
 </discussion>