Fix location of Firefox dbCacheFile

Summary:
Before: ~/.cache/krunnerbookmarkrunnerfirefoxdbfile.sqlite
After: ~/.cache/krunner/bookmarkrunnerfirefoxdbfile.sqlite

My guess is that the slash got lost while porting from KDE4.

Test Plan: KRunner creates the new file and uses it.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10605