Update links in RNA-seq, pathway-analysis, and machine-learning cheatsheets

_From https://github.com/AlexsLemonade/training-modules/issues/515#issuecomment-1058472857_

The following cheatsheets still need updates to match the changes in #521.

- [ ] [RNA-seq-cheatsheet.md](https://github.com/AlexsLemonade/training-modules/blob/master/module-cheatsheets/RNA-seq-cheatsheet.md)
- [ ] [pathway-analysis-cheatsheet.md](https://github.com/AlexsLemonade/training-modules/blob/master/module-cheatsheets/pathway-analysis-cheatsheet.md)
- [ ] [machine-learning-cheatsheet.md](https://github.com/AlexsLemonade/training-modules/blob/master/module-cheatsheets/machine-learning-cheatsheet.md)

I.e., need to adhere to our new contributing guidelines:

> When choosing documentation links to incorporate in cheatsheets, we prefer to use [`https://rdrr.io/`](https://rdrr.io/) when possible for Base R and Bioconductor, and we prefer to use [`https://www.tidyverse.org/`](https://www.tidyverse.org/) for `tidyverse` functions

Start by reviewing the changes in #521 and the contributing guidelines, then inspect module-cheatsheets/RNA-seq-cheatsheet.md, pathway-analysis-cheatsheet.md, and machine-learning-cheatsheet.md. Update documentation links in those three files to prefer rdrr.io for Base R and Bioconductor and tidyverse.org for tidyverse functions; done means all listed links follow those preferences.

Written by the indexing model from the issue text.