update tenant retention proposal doc with implemented status (#4574)

* update tenant retention period doc with new status

Signed-off-by: benjyiw <[email protected]>

* Update tenant_retention.md

Added a period mainly to kick off the workflows again, so I can merge.

Signed-off-by: Alvin Lin <[email protected]>

Co-authored-by: Alvin Lin <[email protected]>

Author: benjyiw

docs/proposals/tenant_retention.md

@@ -7,7 +7,7 @@ slug: tenant-retention
 
 - Author: [Allenzhli](https://github.com/Allenzhli)
 - Date: January 2021
-- Status: Proposed
+- Status: Accepted, Implemented in [PR #3879](https://github.com/cortexproject/cortex/pull/3879).
 
 ## Retention of tenant data