fix typo github webhook event security_vulnerability json field name

[yhamano0312]

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

[coveralls]

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