Update broken link to GitHub docs
Update broken link to GitHub docs

GitHub is now redirecting the link in question (about Github Flavored Markdown) to a generic help index. I found [a page on Github Guides that specifically addresses GFM](https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown). It doesn't have the exact same content but on balance it seems preferable to a [wayback machine link](https://web.archive.org/web/20160123003010/https://help.github.com/articles/github-flavored-markdown/)?
StackExchange/stack-blog,StackExchange/blog,StackExchange/stack-blog,StackExchange/blog,StackExchange/stack-blog,dgrtwo/stack-blog,StackExchange/blog,StackExchange/stack-blog,jericson/stack-blog,dgrtwo/stack-blog,jericson/stack-blog,dgrtwo/stack-blog,StackExchange/blog,jericson/stack-blog,dgrtwo/stack-blog,StackExchange/stack-blog,dgrtwo/stack-blog,StackExchange/blog,jericson/stack-blog,jericson/stack-blog