Editorial: improve HTMLAllCollection IDL

As per whatwg/webidl#532.

Author: annevk

source

@@ -7337,7 +7337,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
  LegacyUnenumerableNamedProperties]
 interface <dfn>HTMLAllCollection</dfn> {
   readonly attribute unsigned long <span data-x="dom-HTMLAllCollection-length">length</span>;
-  getter <span>Element</span>? (unsigned long index);
+  getter <span>Element</span> (unsigned long index);
   getter (<span>HTMLCollection</span> or <span>Element</span>)? <span data-x="dom-HTMLAllCollection-namedItem">namedItem</span>(DOMString name);
   (<span>HTMLCollection</span> or <span>Element</span>)? <span data-x="dom-HTMLAllCollection-item">item</span>(optional DOMString nameOrIndex);