docs: fix dead boolean article link in Clean ABAP DE

[Semak12345]

Summary

• replace the dead boolean article link in the German Clean ABAP guide
• point it to the same archived source already used elsewhere in the repo

Fixes #366

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Sam Kosh seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}