[BUGFIX] Set selected CSS class on image in RTE

Fix a copy&paste error in the javascript to set the selected class
on the image.

Resolves: #73657
Releases: master, 7.6
Change-Id: I8b7e3580185221f98d16c8fd48fe9f9fc21cfd52
Reviewed-on: https://review.typo3.org/46882


Reviewed-by: Harald <eisner.harald@online.de>
Tested-by: Harald <eisner.harald@online.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>