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

[jaclyn-taroni]

From #515 (comment)

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

• RNA-seq-cheatsheet.md
• pathway-analysis-cheatsheet.md
• 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/ when possible for Base R and Bioconductor, and we prefer to use https://www.tidyverse.org/ for tidyverse functions

[sjspielman]

I've started this already, but wanted to get the more immediately-needed cheatsheets done first. I was to circle back to these next sprint most likely, unless we want it done sooner?