Doc: Fix module name used in documentation filter

Qt WebKit Examples creates a filter with name 'QtWebKit'.
Change it to use the correct name so that Qt WebKit can
add its own filter.

Task-number: QTBUG-34966
Change-Id: Iad74a20d72eebf2e20309a54f633a24f8c1e3d12
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>