Fixed wrong path to munin-node config file in doc

plugins/nginx/nginx_upstream_multi_

@@ -25,7 +25,7 @@
 # Use it in your site configuration (/etc/nginx/sites-enabled/anything.conf):
 # access_log /var/log/nginx/upstream.log upstream;
 #
-# And specify some options in munin-node.conf:
+# And specify some options in /etc/munin/plugin-conf.d/munin-node:
 #
 #     [nginx_upstream_multi_upstream]
 #     env.graphs cache http time request