Replace dead link in Security Policy (#274)

Author: Jubeki

.github/SECURITY.md

@@ -15,7 +15,7 @@ If you discover a security vulnerability within Laravel, please send an email to
 ```
 -----BEGIN PGP PUBLIC KEY BLOCK-----
 Version: OpenPGP v2.0.8
-Comment: https://sela.io/pgp/
+Comment: Report Security Vulnerabilities to [email protected]
 
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