Fix R installation instructions for Linux

[gcapes]

https://carpentries.github.io/sandpaper-docs/#r-2

This section suggests to install R from CRAN, but gives no indication that using the version from your package manager is likely to fail because the R version is too old. I would suggest stronger wording and an explanation of why. Perhaps a minimum version number of R that is required?

I also found that after the sudo add-apt-repository step, I needed to update my apt cache. This step is missing from the instructions..

[froggleston]

We can certainly add more improvements to the docs - that's a good suggestion!

Perhaps a minimum version number of R that is required? - sadly this is difficult to state as it completely depends on what packages lessons need, and the versions that are available of those packages. It's quite common to see a minor version update to R that will break a low level package, and the only solution is to wait until the package developer resolves the issue for the newer R version.