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

@@ -128,7 +128,7 @@ can find it at `ISSUE_TEMPLATE.md <https://github.com/openedx/help-tokens/blob/m
 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
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