[BUGFIX] Fix spelling errors in ext:backend

Resolves: #90582
Releases: master
Change-Id: I456dd0b04947c838f125d49ca686a40d289c2fe1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63487


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Susanne Moog <look@susi.dev>