[BUGFIX] Fix typo in identifier exists validation message in site configuration

This commit fixes small typo at the end of the identifier exists validation message
used in site configuration controller.

Resolves: #91314
Releases: master, 9.5
Change-Id: Iccda594038a30a277d1c45255f6de564aa3dda5f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64417


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Joerg Kummer <typo3@enobe.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>