Correct the link of sswg-security at SECURITY.md (#619)

### 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

SECURITY.md

@@ -38,6 +38,6 @@ with the details usually included with bug reports.
   forums][swift-forums-sec].
 
 [sswg]: https://github.com/swift-server/sswg
-[sswg-security]: https://github.com/swift-server/sswg/blob/main/process/incubation.md#security-best-practices
+[sswg-security]: https://www.swift.org/sswg/security/
 [swift-forums-sec]: https://forums.swift.org/c/server/security-updates/
 [mitre]: https://cveform.mitre.org/