Fix the json field name of security_vulnerability in DependabotAlertPayload. The json field name should be security_vulnerability according to https://docs..com/ja/webhooks/webhook-events-and-payloads?actionType=created#dependabot_alert

coverage: 88.933%. remained the same
when pulling acb3837 on yhamano0312:fix--security-vulnerability-tag-name
into 02d182a on go-playground:master.