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

[simonjbeaumont]

Thanks!