[BUGFIX] Clarify description in ExtensionManagementUtility::addPlugin

The description that the extension key should be used - instead of the
plugin identifier is wrong, and is now adapted.

Resolves: #90113
Releases: master, 9.5
Change-Id: I4928dba4255d46bf72e0a0b45e5ff3360ae718ac
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62921


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Steffen Frese <steffenf14@gmail.com>
Tested-by: Felix P. <f.pachowsky@neusta.de>
Tested-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Steffen Frese <steffenf14@gmail.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Felix P. <f.pachowsky@neusta.de>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>