updatesysutils/snmp_exporter: do not overwrite generator.yml

Mark it as sample file.
generator.yml is used by snmp_exporter_generator to generate the final
configuration (snmp.yml) which should not be edited by hand.

PR: 263878
Reported by: topical@gmx.net
Approved by: fabian.freyer@physik.tu-berlin.de (1 week)