[BUGFIX] Invalid phpDoc in ToolbarItemInterface

Method getAdditionalAttributes() returns an array of HTML attributes.

Resolves: #64058
Releases: master
Change-Id: I38022eceffd8eb8d0121046e5323b9f81d56092b
Reviewed-on: http://review.typo3.org/35670
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>