Editorial: make the FormDataEvent constructor's argument required

It already was, effectively, since it had a required member, but this is
more explicit.

Fixes #5064. Also fixes the constructor syntax per #4870.

source

@@ -57117,9 +57117,10 @@ dictionary <dfn>SubmitEventInit</dfn> : <span>EventInit</span> {
 
   <h5>The <code>FormDataEvent</code> interface</h5>
 
-  <pre><code class="idl" data-x="">[Exposed=Window,
- Constructor(DOMString type, optional <span>FormDataEventInit</span> eventInitDict = {})]
+  <pre><code class="idl" data-x="">[Exposed=Window]
 interface <dfn>FormDataEvent</dfn> : <span>Event</span> {
+  constructor(DOMString type, <span>FormDataEventInit</span> eventInitDict);
+
   readonly attribute <span>FormData</span> <span data-x="dom-FormDataEvent-formData">formData</span>;
 };