[BUGFIX] Add missing space in regular expression

DataHandler::clearPrefixFromValue is missing a space
in the regular expression when removing the prependAtCopy
label. That leads to wrong headers in copied elements.

Releases: master
Resolves: #83870
Change-Id: I49cde9a9a20686715d9b042173f89bfeb2bd112f
Reviewed-on: https://review.typo3.org/55694


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>