[BUGFIX] Fix typo in TCA of fe_users

A typo in the definition of the field "usergroup" leads to an entry
in the deprecation log.

Change-Id: I571edd4b6bd6680653f13b9d0de84d4107ff142a
Resolves: #70672
Releases: master
Reviewed-on: http://review.typo3.org/44036


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>