Fixed code blocks in the 3rdparty's readme.
Added a code block in the README.md of /3rdparty.
This does not affect anything else than the 3rdparty's Readme markdown file, so there's nothing to test really, and nothing that could break the build pipeline. Keeping the next section (since it's my first MR here, I don't know how much freedom I have on the formatting here), but there's not much to that merge request to verify.
- I confirmed this builds.
- I confirmed Krita ran and the relevant functions work.
- I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
- I made sure my commits build individually and have good descriptions as per KDE guidelines.
- I made sure my code conforms to the standards set in the HACKING file.
- I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.