doc: Don't set nested headers for nginx, use more_set_headers instead

See https://github.com/yandex/gixy/blob/master/docs/en/plugins/addheaderredefinition.md

doc/tutorial/deployment.rst

@@ -100,7 +100,7 @@ Nginx
             location ~* ^.+\.(?:css|cur|js|jpe?g|gif|htc|ico|png|html|xml|otf|ttf|eot|woff|woff2|svg)$ {
                 access_log off;
                 expires 30d;
-                add_header Cache-Control public;
+                more_set_headers Cache-Control public;
             }
         }