rstudio/cheatsheets changed branch name

[LFenske]

In rstudio.Rmd there is a reference to https://github.com/rstudio/cheatsheets/raw/master/rstudio-ide.pdf that should be changed to https://github.com/rstudio/cheatsheets/raw/main/rstudio-ide.pdf. That is "master" changed to "main". Looks like the same issue is in why-rmd.Rmd.

[njtierney]

Thank you!