Update header.ejs
Update header.ejs

fix bug: if /_config.yml -> root is not default "/", then the background image will failed to load because of url path error.
Bug 修复: 当/_config.yml中root值不是默认的"/"情况下, 背景图片加载错误, 因为路径不正确.
levblanc/hexo-theme-aero-dual