fix headers in CONTRIBUTING.md and SECURITY.md

CONTRIBUTING.md

@@ -1,15 +1,17 @@
-**Contribution Policy**
+# Contribution Policy
 
 Cloud.gov is an open source project operated by the U.S. General Services Administration (GSA) to support federal agency missions. While we value transparency and collaboration, we must balance openness with the responsibilities of operating a secure, compliant, and trusted federal platform.
 
-✅ **Who can contribute**
+## Who can contribute
+
 We welcome contributions from:
 
 - Employees of U.S. federal agencies
 - Contractors working under a current agreement with a U.S. government entity
 - GSA-approved contributors as part of official interagency collaboration
 
-❌ **Who we cannot accept contributions from**
+## Who we cannot accept contributions from
+
 To avoid the appearance of government endorsement, manage supply chain risk, and maintain the integrity of our compliance posture, we do **not** accept unsolicited contributions from:
 
 - Individuals unaffiliated with the U.S. government

SECURITY.md

@@ -1,4 +1,4 @@
-**Reporting Security Issues**
+# Reporting Security Issues
 
 Please refrain from reporting security vulnerabilities through public GitHub issues.