core/types: improve comment on body withdrawals field drop

Co-authored-by: Arran Schlosberg <519948+ARR4N@users.noreply.github.com>
Signed-off-by: Quentin McGaw <quentin.mcgaw@gmail.com>

core/types/block_ext_test.go

@@ -129,7 +129,7 @@ func TestBodyRLP(t *testing.T) {
 
 	wantBody, wantExtra := bodyWithNonZeroFields()
 
-	wantBody.Withdrawals = nil // this should be ignored
+	wantBody.Withdrawals = nil // this should be dropped by the RLP round trip
 	opts := cmp.Options{
 		txHashComparer(),
 		headerHashComparer(),