Fix copypasted comment (#7477)

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>

changelog.d/7477.doc

+Fix copy-paste error in `ServerNoticesConfig` docstring. Contributed by @ptman.
\ No newline at end of file

synapse/config/server_notices_config.py

@@ -51,7 +51,7 @@ class ServerNoticesConfig(Config):
             None if server notices are not enabled.
 
         server_notices_mxid_avatar_url (str|None):
-            The display name to use for the server notices user.
+            The MXC URL for the avatar of the server notices user.
             None if server notices are not enabled.
 
         server_notices_room_name (str|None):