fix: hreflang tag attribute of feed misses site.alt_lang

Author: cotes2020

assets/feed.xml

@@ -15,7 +15,7 @@ permalink: /feed.xml
     <uri>{{ "/" | absolute_url }}</uri>
   </author>
   <link rel="self" type="application/atom+xml" href="{{ page.url | absolute_url }}"/>
-  <link rel="alternate" type="text/html" hreflang="{{ site.lang }}"
+  <link rel="alternate" type="text/html" hreflang="{{ site.alt_lang | default: site.lang }}"
     href="{{ '/' | absolute_url }}"/>
   <generator uri="https://jekyllrb.com/" version="{{ jekyll.version }}">Jekyll</generator>
   <rights> © {{ 'now' | date: '%Y' }} {{ site.social.name }} </rights>