Fix the NumFocus link (#7436)

Found the following (possibly) invalid URLs:
  URL: http://numfocus.org (moved to https://numfocus.org/)
    From: README.md
    Status: 200
    Message: OK

Author: aitap

README.md

@@ -10,7 +10,7 @@
 [![CRAN usage](https://jangorecki.gitlab.io/rdeps/data.table/CRAN_usage.svg?sanitize=true)](https://gitlab.com/jangorecki/rdeps)
 [![BioC usage](https://jangorecki.gitlab.io/rdeps/data.table/BioC_usage.svg?sanitize=true)](https://gitlab.com/jangorecki/rdeps)
 [![indirect usage](https://jangorecki.gitlab.io/rdeps/data.table/indirect_usage.svg?sanitize=true)](https://gitlab.com/jangorecki/rdeps)
-[![Powered by NumFOCUS](https://img.shields.io/badge/powered%20by-NumFOCUS-orange.svg?style=flat&colorA=E1523D&colorB=007D8A )](http://numfocus.org)
+[![Powered by NumFOCUS](https://img.shields.io/badge/powered%20by-NumFOCUS-orange.svg?style=flat&colorA=E1523D&colorB=007D8A )](https://numfocus.org)
 <!-- badges: end -->
 
 `data.table` provides a high-performance version of [base R](https://www.r-project.org/about.html)'s `data.frame` with syntax and feature enhancements for ease of use, convenience and programming speed.