docs: Update the security e-mail address.

[feanil]

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (49c9d44) 56.36% compared to head (d3daf65) 56.36%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   56.36%   56.36%           
=======================================
  Files           3        3           
  Lines          55       55           
=======================================
  Hits           31       31           
  Misses         24       24           

Flag	Coverage Δ
unittests	56.36% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.