Add missing and

Author: lukewarlow

dom.bs

@@ -6560,7 +6560,7 @@ string <var>namespace</var> (default null):</p>
 <div algorithm>
 <p>To <dfn export id=concept-element-attributes-set-value>set an attribute value</dfn> given an
 <a for=/>element</a> <var>element</var>, a string <var>localName</var>, a string or {{TrustedType}}
-<var>value</var>, an optional null or string <var>prefix</var> (default null), an optional null
+<var>value</var>, an optional null or string <var>prefix</var> (default null), and an optional null
 or string <var>namespace</var> (default null):
 
 <ol>