Doc: Fix QtWebEngine version number

The latest version for Qt Webengine in 5.9 will be 1.5, not 1.6.

Change-Id: Ibae42f24f4b7477901923ff0ef5436642599f365
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>