Small typo fix when deleting groups

Created by: teuneboon

When I tried to delete a group I came across a huge issue that I couldn't possibly ignore. I made a group called "testgroup", and this was the message it gave me when I deleted it: Group 'testgroup was deleted.. That weird floating quote is obviously unacceptable for a feature used so often. The message now reads: Group 'testgroup' was successfully deleted.(added "successfully" cause that's also used when updating and creating groups)