Update head.html

referring to the [Google Search Central official documentation](https://developers.google.com/search/docs/specialty/international/localized-versions#xdefault), I specified `x-default` instead of `site.default_lang` as the hreflang attribute value for the site's default language page, so that the link to that page is recognized as a fallback when the visitor's preferred language is not in the list of languages supported by the site or when the visitor's preferred language cannot be recognized.

Author: yunseo-kim

_includes/head.html

@@ -11,7 +11,7 @@
 
   <meta http-equiv="Content-Language" content="{{site.active_lang}}">
 
-  <link rel="alternate" hreflang="{{site.default_lang}}" href="{{site.url}}{{page.url}}" />
+  {% if site.default_lang %}<link rel="alternate" hreflang="x-default" href="{{site.url}}{{page.url}}" />{% endif %}
   {% for lang in site.languages %}{% if lang == site.default_lang %}{% continue %}{% endif %}
   <link rel="alternate" hreflang="{{lang}}" href="{{site.url}}/{{lang}}{{page.url}}" />
   {% endfor %}