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/process/incubation.md#security-best-practices => https://www.swift.org/sswg/security/

Result:

Correct the link of sswg-security

[lamtrinhdev]

Dear @euanh ,

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

Thanks,
Lam

[euanh]

Hi @lamtrinhdev. Thank you very much for your pull request.

In fact this project currently uses the Swift.org security policy, so I've updated this file to link to that. Please take a look at #21.