Fix R CMD examples in the contributing guide

[manuteleco]

R build and check tools are lowercased. Trying to invoke them with uppercased names does not work. For example:

$ R CMD BUILD
/usr/local/lib/R/bin/Rcmd: 64: exec: BUILD: not found

[manuteleco]

From what I can tell, docs/CONTRIBUTING.html should also be generated again to account for these changes. But I'm not sure if this is something I'm supposed to do as part of this PR or not.

I did try running pkgdown::build_site() (if that's event the right thing to do), but it failed with:

✖ Missing topics: googleCloudStorageR

I'm in the process of preparing another PR, where I'm actually adding documentation to vignettes, and I'm also facing the same question/challenge around pkgdown.

Let me know how I should proceed.

[MarkEdmondson1234]

Weird, sure it worked before but all good - thanks