[csharp/de-de] Fix typo, spelling and grammar

[jk4e]

• [x] I solemnly swear that this is all original content of which I am the original author
• [x] Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
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  • [x] Yes, I have double-checked quotes and field names!