Doc: Rename the second qt_add_shaders argument to resource_name

The synopsis called it <name>, which doesn't say what the value is
used for. It is the name of the generated resource, same as for
qt_add_resources().

Pick-to: 6.12
Fixes: QTBUG-149152
Change-Id: Ib6b7b36bb9dae797086e5992825a8e7392022466
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>