[BUGFIX] Followup to merge extra_page_cm_options into backend

Syntax error in line 298

Releases: 6.3
Resolves: #55136
Change-Id: I497e0e2b65b40a87ef1f8af205a0991b4dd92f74
Reviewed-on: http://review.typo3.org/32698
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>