fix typo in MAGE_MOD

nginx.conf.sample

@@ -13,7 +13,7 @@
 #    listen 80;
 #    server_name mage.dev;
 #    set $MAGE_ROOT /var/www/magento2;
-#    set $MAGE_MODE develop;
+#    set $MAGE_MODE developer;
 #    include /vagrant/magento2/nginx.conf.sample;
 # }