Doc: Replaced the HTTPS link 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: I86f13ad2e871bd1aee8ebe39ebc0d8fc26cf2f20
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>