fix(i18n): Fixed grammar

[rakekniven]

Reported at Transifex.

[juliushaertl]

In server we seem to mostly use administrator. Should this be a general adjustment? Was there some discussion around that?

[rakekniven]

While translating Transifex (not only for german) we often had discussion about gender neutral strings.
Actually "administrator" is dominating and I had conversations with Jan (cannot find links).
See use of administration at https://app.transifex.com/nextcloud/search/?q=project%3Anextcloud+target_language%3Ade_DE+source_text%3AAdministration

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