Update Security.md to note that GitHub security advisories can be used

Author: kovidgoyal

SECURITY.md

@@ -7,6 +7,5 @@ and released just like all other bugs.
 
 ## Reporting a vulnerability
 
-Preferably send an email to kovid at kovidgoyal.net or open an issue in the
-GitHub repository, though the latter means you are disclosing the vulnerability
-publicly before it can be fixed.
+Preferably send an email to kovid at kovidgoyal.net or open a private security
+advisory using the GitHub security advisory facility.