fix(background): #776 bing

Author: MHuiG

_config.yml

@@ -113,7 +113,7 @@ cover:
     home: true
     archive: true
     others: false # can be written in front-matter 'cover: true'
-  background: https://uploadbeta.com/api/pictures/random/?key=BingEverydayWallpaperPicture
+  background: https://fastly.jsdelivr.net/gh/MHG-LAB/cron@gh-pages/bing/bing.jpg
   # background: https://bing.ioliu.cn/v1/rand?w=1920&h=1200
   logo: # https://cdn.jsdelivr.net/gh/volantis-x/cdn-org/blog/[email protected]
   title: 'Volantis'