net-analyzer/prometheus-snmp_exporter: fix typo in initd

net-analyzer/prometheus-snmp_exporter: fix typo in initd

austin@austin1 ~ $ touch snmp.yml

austin@austin1 ~ $ /usr/bin/snmp_exporter --config-file=snmp.yml
snmp_exporter: error: unknown long flag '--config-file', try --help

austin@austin1 ~ $ /usr/bin/snmp_exporter --config.file=snmp.yml
INFO[0000] Starting snmp exporter (version=0.8.0, branch=non-git, revision=17cefca)  source="main.go:125"
INFO[0000] Build context (go=go1.9.4, user=portage@releng3, date=20180222-23:34:24)  source="main.go:126"
INFO[0000] Listening on :9116                            source="main.go:205"

Package-Manager: Portage-2.3.19, Repoman-2.3.6