Klipper lib/stm32*: Update STM32 header references.

[bevanweiss]

The README referenced paths in a github that were themselves git submodules.
So have altered README to point to the referenced git repo itself rather than the containing git repo.

[github-actions]

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[KevinOConnor]

Alas, the STM provided code is of low-quality. The code often has bugs, they don't publish meaningful changelogs, they frequently change the license statements (often making them more ambiguous), and they have a history of changing the repo locations.

As near as I can tell the current Klipper lib/README file is accurate (one can obtain the code from the given repos by checking out the specified commit). Until that changes or some new functionality requires an update, I would avoid "chasing" STM's changes.

Thanks,
-Kevin