[execution.syn] Rename template parameter of schedule_result_t (#7906)

Author: ericniebler

source/exec.tex

@@ -611,8 +611,8 @@
   inline constexpr schedule_t @\libglobal{schedule}@{};
   inline constexpr @\unspec@ @\libglobal{read_env}@{};
 
-  template<@\libconcept{scheduler}@ Sndr>
-    using @\libglobal{schedule_result_t}@ = decltype(schedule(declval<Sndr>()));
+  template<@\libconcept{scheduler}@ Sch>
+    using @\libglobal{schedule_result_t}@ = decltype(schedule(declval<Sch>()));
 
   // \ref{exec.adapt}, sender adaptors
   template<@\exposconcept{class-type}@ D>