Handle old value correctly when changing an attribute

This fixes an error just introduced in 3fb0aa6.

Author: dbaron

dom.bs

@@ -6297,10 +6297,12 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
 <a>attribute</a> <var>attribute</var> to <var>value</var>, run these steps:
 
 <ol>
+ <li><p>Let <var>oldValue</var> be <var>attribute</var>'s <a for=Attr>value</a>.</p></li>
+
  <li><p>Set <var>attribute</var>'s <a for=Attr>value</a> to <var>value</var>.
 
  <li><p><a>Handle attribute changes</a> for <var>attribute</var> with <var>attribute</var>'s
- <a for=Attr>element</a>, <var>attribute</var>'s <a for=Attr>value</a>, and <var>value</var>.
+ <a for=Attr>element</a>, <var>oldValue</var>, and <var>value</var>.
 </ol>
 
 <p>To <dfn export id=concept-element-attributes-append lt="append an attribute">append</dfn> an