[exec.snd.expos] Mark valid-specializations with \exposconcept (#7557)

Author: ericniebler

source/exec.tex

@@ -1630,7 +1630,7 @@
   };
 
   template<class Sndr, class Rcvr, class Index>
-    requires @\exposid{valid-specialization}@<@\exposid{env-type}@, Index, Sndr, Rcvr>
+    requires @\exposconcept{valid-specialization}@<@\exposid{env-type}@, Index, Sndr, Rcvr>
   struct @\exposid{basic-receiver}@ {                                       // \expos
     using receiver_concept = receiver_t;