screenshot-page: rm `container` attr on `#content`

It is invalid HTML.
I added it with SASS-ZERO.
I don't remember if it was necessary for SASS-ZERO.

screenshot-page.html

@@ -18,7 +18,7 @@
   -->
 
   <main>
-    <section id="content" container class="container">
+    <section id="content" class="container">
       <article>
         <h1>Lorem ipsum dolor sit amet</h1>
         <h2>Element demos</h2>