Fix the toggle_R() example.

See example: 
It appears pandoc no longer uses class "sourceCode r", but rather just 'r'. 

Rpubs: http://rpubs.com/Kevin-M-Smith/39263

View source at this page to see javascript: http://rstudio-pubs-static.s3.amazonaws.com/39263_d9bc838e057749b9a8c98613ebd406f5.html
Merge pull request #888 from Kevin-M-Smith/patch-1

Fix the toggle_R() example.