[applets/taskmanager] Revert "forceStripes" checkbox string change

Summary:
I changed the "forceStripes" checkbox string in D19280, most likey because I thought it wasn't super understandable.

The new string I pushed doesn't, however, match what the option does.

So reverting the change.

BUG: 412801

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24534