Update sites.md
Update sites.md

Link has changed to GitHub's training materials.
xcatliu/jekyllcn,fengsmith/jekyllcn,zhangkuaiji/jekyllcn,zhangkuaiji/jekyllcn,penchan1218/jekyllcn,fengsmith/jekyllcn,honger05/jekyllcn,fengsmith/jekyllcn,UniFreak/jekyllcn,LeuisKen/jekyllcn,zhangkuaiji/jekyllcn,xcatliu/jekyllcn,LeuisKen/jekyllcn,honger05/jekyllcn,UniFreak/jekyllcn,honger05/jekyllcn,xcatliu/jekyllcn,UniFreak/jekyllcn,penchan1218/jekyllcn,LeuisKen/jekyllcn,xcatliu/jekyllcn,penchan1218/jekyllcn