Fix crash in d-figure

examples/article.html

@@ -23,7 +23,7 @@
 <d-article>
   <d-title>
     <h1>Attention and Augmented Recurrent Neural Networks</h1>
-    <!-- <h2>Some people want a deck</h2> -->
+    <h2>Some people want a deck</h2>
     <d-byline></d-byline>
   </d-title>
   <d-abstract>

src/components/d-figure.js

@@ -99,7 +99,7 @@ export class Figure extends HTMLElement {
 
   ready() {
     this._ready = true;
-    this.marginObserver.unobserve(this);
+    Figure.marginObserver.unobserve(this);
     const event = new CustomEvent('ready');
     this.dispatchEvent(event);
   }