Fix RSS link for paginated list pages (adityatelange#1192)

The link was previously hard-coded to index.html, which doesn't work on
paginated list pages because it would go to something like
/section/pages/2/index.html instead of /section/index.html.

layouts/_default/list.html

@@ -10,7 +10,8 @@
   <h1>
     {{ .Title }}
     {{- if and (or (eq .Kind `term`) (eq .Kind `section`)) (.Param "ShowRssButtonInSectionTermList") }}
-    <a href="index.xml" title="RSS" aria-label="RSS">
+    {{- with .OutputFormats.Get "rss" }}
+    <a href="{{ .RelPermalink }}" title="RSS" aria-label="RSS">
       <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"
         stroke-linecap="round" stroke-linejoin="round" height="23">
         <path d="M4 11a9 9 0 0 1 9 9" />
@@ -19,6 +20,7 @@ <h1>
       </svg>
     </a>
     {{- end }}
+    {{- end }}
   </h1>
   {{- if .Description }}
   <div class="post-description">