[exec.associate] Use \exposid for "voidify", not \placeholdernc

Author: eisenwave

source/exec.tex

@@ -5119,7 +5119,7 @@
     explicit op_state(pair<assoc_t, sender_ref_t> parts, Rcvr& r)
       : @\exposid{assoc}@(std::move(parts.first)) {
       if (@\exposid{assoc}@) {
-        ::new (@\placeholdernc{voidify}@(@\exposid{op}@)) op_t(connect(std::move(*parts.second), std::move(r)));
+        ::new (@\exposid{voidify}@(@\exposid{op}@)) op_t(connect(std::move(*parts.second), std::move(r)));
       } else {
         @\exposid{rcvr}@ = addressof(r);
       }