🔗 Fix link to datapackages-r

[nepito]

The link to datapackages.r is broken.

This new link refers to a repository datapackages-r from the frictionlessdata organization.

[peterdesmet]

Thanks! While https://cran.r-project.org/package=datapackage.r is the canonical way to refer to the package, the GitHub link is more useful to users as it provides documentation and instructions to install from GitHub. The name should remain datapackage.r (that is the official name as defined in https://github.com/frictionlessdata/datapackage-r/blob/master/DESCRIPTION#L1).

Will update to use the new link (and same name) in README.Rmd (the source of README.md)