docs: use mailto scheme for security email in SECURITY.md

[FirmaSpring]

Problem

SECURITY.md still writes the security contact as:

Please email us at [security@hangfire.io](security@hangfire.io) with details

Without a mailto: scheme this renders as a relative URL. Verified with a GET:

curl -sL -o /dev/null -w "%{http_code} %{url_effective}\n" \
  "https://github.com/HangfireIO/Hangfire/blob/main/security@hangfire.io"
# 404

So the rendered link leads to a 404 page instead of opening a mail client.

README.md already received the mailto: prefix in 8c2e32e; SECURITY.md was left out.

Solution

Use mailto:security@hangfire.io as the link target in SECURITY.md, matching the already-fixed README.

Verification

Markdown-only change. Checked open and closed PRs for prior mailto/security-link fixes on SECURITY.md and found none.

[odinserj]

Thank you @FirmaSpring!