doc: More fixes for repogen documentation

 - Remove '-c config' from docs
 - Remove '--updateurl' from docs

Change-Id: I2a7a6d3ffd370527ab3d72a2db7c120355f1931f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>