[BUGFIX] Fix selector of "Add more lines"

Due to latest changes, ``this`` refers to document now. Change the
selector to fix the event binding.

Resolves: #70935
Releases: master
Change-Id: Ic98329bb19c4ebea80b66f59eed2c1c95427811f
Reviewed-on: https://review.typo3.org/44212


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>