Added aria-label to default.hbs' burger menu

no issue

- Improve accessibility
- Improve SEO (by improving accessibility)

default.hbs

@@ -36,7 +36,7 @@
                     {{/if}}
                 </a>
                 <button class="gh-search gh-icon-btn" aria-label="Search this site" data-ghost-search>{{> "icons/search"}}</button>
-                <button class="gh-burger"></button>
+                <button class="gh-burger" aria-label="Main Menu"></button>
             </div>
 
             <nav class="gh-head-menu">