Use s to deliminate R code blocks

Github's syntax highlighting does not correctly use the 's' lexer to highlight markdown blocks delimited with R or r, you have to use s explicitly.  I attempted to fix the issue https://github.com/github/linguist/issues/881 but the bug is apparently somewhere else, perhaps in the pygments library.

As long as the bug exists, you can get proper R highlighting by delimiting your code blocks with ```s instead, which is what this commit does :)