Add return type to createButton for WebVR typings.

The function can return either a button or an `<a>` element. `HTMLElement` is a simple generalization in case the implementation changes again.

examples/jsm/vr/WebVR.d.ts

@@ -7,5 +7,5 @@ export interface WEBVROptions {
 }
 
 export namespace WEBVR {
-	export function createButton( renderer: WebGLRenderer, options?: WEBVROptions );
+	export function createButton( renderer: WebGLRenderer, options?: WEBVROptions ): HTMLElement;
 }