[BUGFIX] Fix typo in ext:backend fluid template

Resolves: #79202
Releases: master, 7.6
Change-Id: Ie47e6b42a7429ce75a8d750c7e7aa7f1c1cb9bec
Reviewed-on: https://review.typo3.org/51201


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>