docs: Update the security e-mail address.

This repository is now managed by the Axim Collaborative and security issues
with it should be reported to security@openedx.org instead of security@edx.org

This work is being done as a part of openedx/wg-security#16

README.rst

@@ -84,7 +84,7 @@ can find it at `ISSUE_TEMPLATE.md <.github/ISSUE_TEMPLATE.md>`_.
 Reporting Security Issues
 -------------------------
 
-Please do not report security issues in public. Please email security@edx.org.
+Please do not report security issues in public. Please email security@openedx.org.
 
 Getting Help
 ------------