Add missing bracket to nginx config (decred#443)

sample-nginx.conf

@@ -54,7 +54,7 @@ http {
         add_header                      Strict-Transport-Security max-age=15552001;
 
         # Serve static resources directly.
-        location /assets/*
+        location /assets/* {
                 gzip_static on; # use .gz files for pre-compressed data
                 root /opt/dcrstakepool/public;
                 # Set the Cache-Control and Expires headers for the static assets.