chore: Fix typos and grammar errors (german language)

[stefankluempers]

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Fixed several typos and grammar errors in german message properties-file.

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This pull request is approved and it will be merged shortly. Thank you very much! 🎉🎉

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Congratulations on merging your first pull request here! 🎉 Excellent work! Keep'em coming. Just in case you have not done so already, the Apereo CAS Contributor Guidelines are available here for your review: https://apereo.github.io/cas/developer/Contributor-Guidelines.html If there is anything unclear about process, or if you would like to see additional information on a particular topic or workflow, please share your feedback with the project. Once again, we appreciate your time and sincere effort and hope to see more from you!

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