There are no distinct commercial and opensource source archives anymore.
Some people do also build from git, so adding the archive name
here is just misleading. Given that users read the README, they arguably
should know already where the sources are ...

Change-Id: I42aac84cd0c904b7b6f2dbb19cacf8b3021f6b21
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>