Fix the webroot method in the nginx config

installation/pleroma.nginx

@@ -22,7 +22,7 @@ server {
     # to get the certificate, and then uncomment it.
     #
     # location ~ /\.well-known/acme-challenge {
-    #     alias /var/lib/letsencrypt/.well-known/acme-challenge;
+    #     root /var/lib/letsencrypt/;
     # }
     location / {
       return         301 https://$server_name$request_uri;