Use $host instead of $http_host in nginx config (#2403)

`$http_host` is vulnerable to host spoofing. Use `$host` instead. See
f5a54ad161/docs/en/plugins/hostspoofing.md

docs/nginx.md

@@ -31,7 +31,7 @@ server {
 
   location / {
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    proxy_set_header Host $http_host;
+    proxy_set_header Host $host;
 
     # If the file exists as a static file serve it directly without
     # running all the other rewrite tests on it