This is a re-submission to fix invalid URL.

## Test environments

* local mac OS install, R 4.0.2
* ubuntu 20.04 (on github actions), R-devel, R 4.0.2, R 3.6.3, 3.5.3, R 3.4.4, R 3.3.3
* mac OS 10.15.4 (on github actions) R-devel, R 4.0.2
* Microsoft Windows Server 2019 10.0.17763 (on github actions) R 4.0.2

## R CMD check results

0 errors v | 0 warnings v | 0 notes v

R CMD check succeeded

## revdepcheck results

No worsening reverse dependencies.