Update common Prometheus files

Signed-off-by: prombot <prometheus-team@googlegroups.com>

CODE_OF_CONDUCT.md

@@ -1,3 +1,3 @@
-## Prometheus Community Code of Conduct
+# Prometheus Community Code of Conduct
 
-Prometheus follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+Prometheus follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).

SECURITY.md

@@ -3,4 +3,4 @@
 The Prometheus security policy, including how to report vulnerabilities, can be
 found here:
 
-https://prometheus.io/docs/operating/security/
+<https://prometheus.io/docs/operating/security/>