Update jellyfin.md example (#598)

* Update jellyfin.md example

The LdapAdminBaseDN either doesn't work properly or is used incorrectly here.

This change will make it work.

see: 
- https://github.com/jellyfin/jellyfin-plugin-ldapauth/issues/145

* Update jellyfin.md

Added some more detail about admin groups and user groups.

example_configs/jellyfin.md

@@ -37,9 +37,9 @@ Otherwise, just use:
 ```
 ### Admin Base DN
 
-The DN of your admin group. If you have `media_admin` as your group you would use:
+The DN to search for your admins.
 ```
-cn=media_admin,ou=groups,dc=example,dc=com
+ou=people,dc=example,dc=com
 ```
 
 ### Admin Filter
@@ -49,6 +49,7 @@ that), use:
 ```
 (memberof=cn=media_admin,ou=groups,dc=example,dc=com)
 ```
+Bear in mind that admins must also be a member of the users group if you use one.
 
 Otherwise, you can use LLDAP's admin group:
 ```