Fix example of output element to use ids instead of names

The first example of the output element incorrectly referenced input
elements in the "for" attribute by their names. This changes the
example to reference input elements by their ids instead (whatwg#3312).

source

@@ -51381,9 +51381,9 @@ interface <dfn>HTMLOutputElement</dfn> : <span>HTMLElement</span> {
    <p>A simple calculator could use <code>output</code> for its display of calculated results:</p>
 
    <pre>&lt;form onsubmit="return false" oninput="o.value = a.valueAsNumber + b.valueAsNumber">
- &lt;input name=a type=number step=any> +
- &lt;input name=b type=number step=any> =
- &lt;output name=o for="a b">&lt;/output>
+ &lt;input id="number-a" name=a type=number step=any> +
+ &lt;input id="number-b" name=b type=number step=any> =
+ &lt;output name=o for="number-a number-b">&lt;/output>
 &lt;/form></pre>
 
   </div>
@@ -121036,6 +121036,7 @@ INSERT INTERFACES HERE
   James Perrett,
   James Robinson,
   Jamie Lokier,
+  Jan K&uuml;hle, <!-- frigus02 on GitHub -->
   Jan Miksovsky,
   Janusz Majnert,
   Jan-Klaas Kollhof,