fixup: state update note

Co-authored-by: Chris Hibbert <[email protected]>

main/guides/zoe/contract-upgrade.md

@@ -156,8 +156,7 @@ const RoomI = M.interface('Room', {
 - Once the state is defined by the `init` function (3rd arg), properties cannot be added or removed.
 - Values of state properties must be serializable.
 - Values of state properties are hardened on assignment.
-- You can replace the value of a state property (e.g. `state.zot = [...state.zot, 'last']`), and you can update stores (`state.players.set(1, player1)`), but you cannot do things like `state.zot.push('last')` nor `state.jot = { x: 1 };`  
-  `state.jot.x = 2;`.
+- You can replace the value of a state property (e.g. `state.zot = [...state.zot, 'last']`), and you can update stores (`state.players.set(1, player1)`), but you cannot do things like `state.zot.push('last')`, and if jot is part of state (`state.jot = { x: 1 };`), then you can't do `state.jot.x = 2;`
 - The tag (1st arg) is used to form a key in `baggage`, so take care to avoid collisions. `zone.subZone()` may be used to partition namespaces.
 - See also [defineExoClass](https://endojs.github.io/endo/functions/_endo_exo.defineExoClass.html) for further detail `zone.exoClass`.
 - To define multiple objects that share state, use `zone.exoClassKit`.