doc: Remove '-u http://www.example.com:8080' from the repogen help

The --updateurl functionality was removed by 17401372c003949a86540c7581e830b980606b73
so removing its usage from the repogen tool help.

Change-Id: Iaf0e7bb65b5f116e5c5e3b921d072db70e8e7236
Reviewed-by: Niels Weber <niels.weber@digia.com>