[BUGFIX] Fix CSS-path to default .t3-icon image

There is no such icon in
typo3/sysext/backend/Resources/Public/gfx/

Usually the default-icon is overwritten in CSS
anyhow by specifying the icon to use, so the
bug does not show. But the path is still wrong.

Change-Id: Ie321cecfb888664a3ec782933e72cfa89e6f41ec
Resolves: #55799
Releases: 6.2
Reviewed-on: https://review.typo3.org/27494
Tested-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring