readme: fix broken security link

Firecracker moved their security policy document to SECURITY.md so that
GitHub would recognize the project security policy automatically.  See
firecracker-microvm/firecracker@ce1ee59.

Fixes #283

Signed-off-by: Samuel Karp <[email protected]>

Author: samuelkarp

README.md

@@ -160,7 +160,7 @@ to report problems, discuss roadmap items, or make feature requests.
 If you've discovered an issue that may have security implications to
 users or developers of this software, please do not report it using
 GitHub issues, but instead follow
-[Firecracker's security reporting guidelines](https://github.com/firecracker-microvm/firecracker/blob/master/SECURITY-POLICY.md).
+[Firecracker's security reporting guidelines](https://github.com/firecracker-microvm/firecracker/blob/master/SECURITY.md).
 
 Other discussion: For general discussion, please join us in the
 `#general` channel on the [Firecracker Slack](https://tinyurl.com/firecracker-microvm).