Update _layouts/default.html to preserve aria-labelledby in headings

Correction suggested by @allejo in comment https://github.com/pmarsceill/just-the-docs/pull/399#discussion_r464128713.
Also prevents losing the aria-labelledby attribute in future updates to this snippet.

_layouts/default.html

@@ -101,7 +101,7 @@ layout: table_wrappers
       {% endunless %}
       <div id="main-content" class="main-content" role="main">
         {% if site.heading_anchors != false %}
-          {% include vendor/anchor_headings.html html=content beforeHeading="true" anchorBody="<svg viewBox=\"0 0 16 16\" aria-hidden=\"true\"><use xlink:href=\"#svg-link\"></use></svg>" anchorClass="anchor-heading" %}
+          {% include vendor/anchor_headings.html html=content beforeHeading="true" anchorBody="<svg viewBox=\"0 0 16 16\" aria-hidden=\"true\"><use xlink:href=\"#svg-link\"></use></svg>" anchorClass="anchor-heading" anchorAttrs="aria-labelledby=\"%html_id%\"" %}
         {% else %}
           {{ content }}
         {% endif %}