QtIFW

Doc: Replace 'Qt Installers' with just 'Installers'

It's obvious from the context that this is about installers created
with the Qt Installer Framework.

Change-Id: If2243f721142b7362c8c718a3c49c45bf288e972
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>