Remove double space in opacity, flow and size sliders on main toolbar

The sliders had two spaces (e.g. ‘Opacity: 75%’) instead of one
(‘Opacity: 75%’). The extra space seems to be added by accident.
They were at least not present in the original review board request
(https://git.reviewboard.kde.org/r/122946/), only in the actual commit
(6d6f154a01ba4d6892f4395736c7b9c304d0b997).