polygon/heimdall: fix errors.Is typo for ErrNotInMilestoneList (#11812)

fixing typo in previous PR
#11808

polygon/heimdall/service.go

@@ -115,9 +115,10 @@ func newCheckpointFetcher(client HeimdallClient, logger log.Logger) entityFetche
 func newMilestoneFetcher(client HeimdallClient, logger log.Logger) entityFetcher[*Milestone] {
 	fetchEntity := func(ctx context.Context, number int64) (*Milestone, error) {
 		milestone, err := client.FetchMilestone(ctx, number)
-		if errors.Is(ErrNotInMilestoneList, err) {
+		if errors.Is(err, ErrNotInMilestoneList) {
 			// this is needed because there may be an unfortunate edge case where FetchFirstMilestoneNum returned 10
 			// but by the time our request reaches heimdall milestone=10 has been already pruned
+			// also we've been observing this error happening sporadically for the latest milestone
 			return milestone, fmt.Errorf("%w: %w", errTransientEntityFetcherFailure, err)
 		}
 		return milestone, err