[exec.prop] Add \libconcept for sender (#8979)

Author: hewillk

source/exec.tex

@@ -6613,7 +6613,7 @@
 \begin{example}
 \begin{codeblock}
 template<@\libconcept{sender}@ Sndr>
-sender auto parameterize_work(Sndr sndr) {
+@\libconcept{sender}@ auto parameterize_work(Sndr sndr) {
   // Make an environment such that \tcode{get_allocator(env)} returns a reference to a copy of \tcode{my_alloc\{\}}.
   auto e = prop(get_allocator, my_alloc{});