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

@@ -101,7 +101,7 @@ Reporting Security Issues
 -------------------------
 
 Please do not report security issues in public. Please email
-security@edx.org.
+security@openedx.org.
 
 .. _individual contributor agreement: https://open.edx.org/wp-content/uploads/2019/01/individual-contributor-agreement.pdf
 .. _CONTRIBUTING: https://github.com/openedx/.github/blob/master/CONTRIBUTING.md