Update site-administration.md

[leishaofa]

在nginx环境下没有任何的说明，这里主要补充一下nginx环境下的配置说明

[kaitlinnewson]

Hi @leishaofa, thanks for your pull request! Do you have any additional description available for what needs to be setup to enable this in nginx, or where exactly this change needs to be made in the config file? For example, for Apache, we say:

"Apache servers use the mod_rewrite plugin, which must be enabled in your httpd.conf file, and the following section added to the correct section of either your httpd.conf file or an .htaccess file (preferred) in your OJS root directory (the same location as config.inc.php):"