Convert CRAN links to preferred, canonical form

https://cran.r-project.org/doc/manuals/r-patched/R-exts.html#Specifying-URLs

Author: katrinleinweber

_includes/links.md

@@ -7,9 +7,9 @@
 [concept-maps]: https://carpentries.github.io/instructor-training/05-memory/
 [contrib-covenant]: https://contributor-covenant.org/
 [contributing]: {{ site.github.repository_url }}/blob/gh-pages/CONTRIBUTING.md
-[cran-checkpoint]: https://cran.r-project.org/web/packages/checkpoint/index.html
-[cran-knitr]: https://cran.r-project.org/web/packages/knitr/index.html
-[cran-stringr]: https://cran.r-project.org/web/packages/stringr/index.html
+[cran-checkpoint]: https://cran.r-project.org/package=checkpoint
+[cran-knitr]: https://cran.r-project.org/package=knitr
+[cran-stringr]: https://cran.r-project.org/package=stringr
 [email]: mailto:[email protected]
 [github-importer]: https://import.github.com/
 [importer]: https://github.com/new/import