presenceReducer [nfc]: Point to the right Flow bug in two fixmes.

These obviously didn't get fixed in the RN v0.63 upgrade, as we'd
thought they would...

It turns out that it's a very similar bug [1] to the one we
originally thought it was [2], but slightly different.
`UserPresence` is an object type with an indexer *and* an additional
property.

[1] facebook/flow#8276
[2] facebook/flow#8178

src/presence/presenceReducer.js

@@ -43,11 +43,15 @@ export default (state: PresenceState = initialState, action: Action): PresenceSt
         ...state,
         [action.email]: {
           ...state[action.email],
-          // $FlowFixMe - Flow bug; should resolve in #4245
+          // Flow bug (unresolved):
+          // https://github.com/facebook/flow/issues/8276
+          // $FlowFixMe
           ...action.presence,
           aggregated: getAggregatedPresence({
             ...state[action.email],
-            // $FlowFixMe - Flow bug; should resolve in #4245
+            // Flow bug (unresolved):
+            // https://github.com/facebook/flow/issues/8276
+            // $FlowFixMe
             ...action.presence,
           }),
         },