Update purge_api README

This points the reverse links at the intended location.

contrib/purge_api/README.md

@@ -3,7 +3,7 @@ Purge history API examples
 
 # `purge_history.sh`
 
-A bash file, that uses the [purge history API](/docs/admin_api/README.rst) to 
+A bash file, that uses the [purge history API](/docs/admin_api/purge_history_api.rst) to 
 purge all messages in a list of rooms up to a certain event. You can select a 
 timeframe or a number of messages that you want to keep in the room.
 
@@ -12,5 +12,5 @@ the script.
 
 # `purge_remote_media.sh`
 
-A bash file, that uses the [purge history API](/docs/admin_api/README.rst) to 
+A bash file, that uses the [purge history API](/docs/admin_api/purge_history_api.rst) to 
 purge all old cached remote media.