fix missing closing tag in sitemapindex.xml

- distopia sitemap entry was missing closing </loc>
- should have caught it in review of #201

Author: orbeckst

sitemapindex.xml

@@ -27,6 +27,6 @@ xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
     <loc>https://www.mdanalysis.org/pytng/sitemap.xml</loc>
   </sitemap> 
   <sitemap>
-    <loc>https://www.mdanalysis.org/distopia/sitemap.xml   
+    <loc>https://www.mdanalysis.org/distopia/sitemap.xml</loc>   
   </sitemap> 
 </sitemapindex>