Doc: Replaced the HTTPS links with HTTP

The HTTPS links fail in Qt Assistant on Windows as the
qt installation package includes qt libraries that are
built without SSL support for legal reasons.

Task-number: QTBUG-31073
Change-Id: I617202d18372b6c454f65251875e5cd0d27a6803
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>