Welcome: fix link to mobile app tutorial

The tutorial filename was changed.

Task-number: QTCREATORBUG-12247

Change-Id: Ibb2733e41807757c2ce435a0b918eada4da99665
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>