[party] Fix bad log line (#37750)

This log line was printing an incorrect new state, and that led me down a multi-hour garden path whereby I was fixing a bug that did not exist.

Closes #37750

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37750 from ctiller:flake-fightas-12 020dd7379c
PiperOrigin-RevId: 676253730

src/core/lib/promise/party.cc

@@ -322,7 +322,7 @@ void Party::RunPartyAndUnref(uint64_t prev_state) {
             (prev_state & (kRefMask | keep_allocated_mask)) - kOneRef,
             std::memory_order_acq_rel, std::memory_order_acquire)) {
       LogStateChange("Run:End", prev_state,
-                     prev_state & (kRefMask | kAllocatedMask) - kOneRef);
+                     (prev_state & (kRefMask | keep_allocated_mask)) - kOneRef);
       if ((prev_state & kRefMask) == kOneRef) {
         // We're done with the party.
         PartyIsOver();