Fix the security-alert email link in the README

This fixes the security-alert email link to actually open up in an email
client, rather than it trying to make the email link a part of the Unit
repository URL.

Fixes: fa42d85 ("Adding GitHub-styled README and CONTRIBUTING files in
Markdown.")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

README.md

@@ -192,7 +192,7 @@ usability.
 - To reach the team directly, subscribe to the
   [mailing list](https://mailman.nginx.org/mailman/listinfo/unit).
 
-- For security issues, [email us](security-alert@nginx.org), mentioning
-  NGINX Unit in the subject and following the [CVSS
+- For security issues, [email us](mailto:security-alert@nginx.org),
+  mentioning NGINX Unit in the subject and following the [CVSS
   v3.1](https://www.first.org/cvss/v3.1/specification-document) spec.