Call set_avatar_url with target_user, not user_id (#8872)

* Call set_avatar_url with target_user, not user_id

Fixes https://github.com/matrix-org/synapse/issues/8871



* Create 8872.bugfix

* Update synapse/rest/admin/users.py

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Testing

* Update changelog.d/8872.bugfix

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>