change security file to markdown (apple#347)

SECURITY.adoc → SECURITY.md

@@ -1,13 +1,13 @@
-= Security
+# Security
 
 For the protection of our community, the Pkl team does not disclose, discuss, or confirm security issues until our investigation is complete and any necessary updates are generally available.
 
-== Reporting a security vulnerability
+## Reporting a security vulnerability
 
 If you have discovered a security vulnerability within the Pkl project, please report it to us.
 We welcome reports from everyone, including security researchers, developers, and users.
 
-Security vulnerabilities may be reported on the link:https://security.apple.com/submit[Report a vulnerability] form.
+Security vulnerabilities may be reported on the [Report a vulnerability](https://security.apple.com/submit) form.
 When submitting a vulnerability, select "Apple Devices and Software" as the affected platform, and "Open Source" as the affected area.
 
 For more information, see https://pkl-lang.org/security.html.