Doc: Fix filter name for Assistant

Use bookcase for the Active Qt documentation filter, like
the reset of the modules do.

Change-Id: I4de24a79308d877dacb146704adbff651af0be5b
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: Iecfdb8ade389de76c49e556dc4d8925c9100dd13