Correct the link of sswg-security at SECURITY.md

[lamtrinhdev]

Motivation:

Correct the link of sswg-security at SECURITY.md. Currently it is showing "Not Found".

Modifications:

• https://github.com/swift-server/sswg/blob/main/security/README.md => https://www.swift.org/sswg/security/

Result:

Correct the link of sswg-security

[lamtrinhdev]

Dear @ktoso , @yim-lee ,

I have a minor update. Could you please help me to review it?

Thanks, Lam

[ktoso]

Good catch thank you