Fix nested main tags breaking sidebar layout

Replace inner <main class="newsletter-main"> with <div> to avoid
invalid nested <main> elements inside the layout's <main id="content">.

Author: pubino

_layouts/newsletter-archive.html

@@ -5,7 +5,7 @@
 <h1>{{ page.title }}</h1>
 
 <div class="newsletter-page">
-  <main class="newsletter-main">
+  <div class="newsletter-main">
     <p><a href="/newsletter/">&larr; All Newsletters</a>{% if page.year and page.month %} | <a href="/newsletter/{{ page.year }}/">{{ page.year }}</a>{% endif %}</p>
 
     <ul class="newsletter-archive">
@@ -28,7 +28,7 @@ <h1>{{ page.title }}</h1>
       {% endif %}
     {% endfor %}
     </ul>
-  </main>
+  </div>
 
   <aside class="newsletter-sidebar">
     <h3>Browse by Year</h3>

newsletter/index.html

@@ -7,7 +7,7 @@
 <h1>SmartDrivingCars Newsletter</h1>
 
 <div class="newsletter-page">
-  <main class="newsletter-main">
+  <div class="newsletter-main">
     <h2>Recent Issues</h2>
     <ul class="newsletter-archive">
     {% assign issues = site.newsletters | sort: 'date' | reverse %}
@@ -17,7 +17,7 @@ <h2>Recent Issues</h2>
       </li>
     {% endfor %}
     </ul>
-  </main>
+  </div>
 
   <aside class="newsletter-sidebar">
     <p class="newsletter-actions">