[mod] libgen: update the URL to http://libgen.rs/

https://libgen.is actually redirect to http://libgen.rs/

It seems there is no HTTPS version:
* https://www.wikidata.org/wiki/Q22017206
* https://librarygenesis.net/

searx/settings.yml

@@ -490,7 +490,7 @@ engines:
 
   - name : library genesis
     engine : xpath
-    search_url : https://libgen.is/search.php?req={query}
+    search_url : http://libgen.rs/search.php?req={query}
     url_xpath : //a[contains(@href,"bookfi.net")]/@href
     title_xpath : //a[contains(@href,"book/")]/text()[1]
     content_xpath : //td/a[1][contains(@href,"=author")]/text()