Update guide.md

> devtools::install_github('rstudio/shinyapps')
Installing github repo(s) rstudio/shinyapps/master from hadley
Downloading rstudio/shinyapps.zip from https://github.com/hadley/rstudio/shinyapps/archive/master.zip
Error: client error: (404) Not Found
> devtools::install_github('shinyapps', 'rstudio')
It works!