Prevent iFrame from instantiating existentials under updates.

I think these cases were accidentally left out in !1035 (merged).

We've only just managed to get to most of our iris proofs in our 8.19 bump and didn't notice breakage from this omission earlier.

/cc @snyke7

iris/base_logic/lib/proph_map.v

@@ -107,7 +107,7 @@ Section proph_map.
     rewrite proph_unseal /proph_def.
     iMod (ghost_map_insert p (proph_list_resolves pvs p) with "H●") as "[H● H◯]".
     { apply not_elem_of_dom. set_solver. }
-    iFrame. iPureIntro. split.
+    iModIntro. iFrame. iPureIntro. split.
     - apply resolves_insert; first done. set_solver.
     - rewrite dom_insert. set_solver.
   Qed.