Fix cheatsheet links

[teodorlu]

Source for cheat sheet links: https://ocamlpro.github.io/ocaml-cheat-sheets/ (via #1561)

Issue Description

Old cheatsheet links were broken. This PR fixes the links.

Fixes #1561

Changes Made

Updated all markdown links to working PDF targets.

• Please check if the PR fulfills these requirements

• ❗ If the PR changes a markdown document in site/, a comment was added in https://github.com/ocaml/ood/issues/52 with a link to the PR
• PR is descriptively titled and links the original issue above
• [N/A] Before/after screenshots (if this is a layout change)
• [N/A] Details of which platforms the change was tested on (if this is a browser-specific change)
• [c] Context for what motivated the change (if this is a change to some content)

[patricoferris]

Thanks for the fix!