[execution.syn] Fix read_env name (#7169)

Author: Eelis

source/exec.tex

@@ -595,7 +595,7 @@
   inline constexpr just_error_t just_error{};
   inline constexpr just_stopped_t just_stopped{};
   inline constexpr schedule_t schedule{};
-  inline constexpr unspecified read{};
+  inline constexpr @\unspec@ read_env{};
 
   template<@\libconcept{scheduler}@ Sndr>
     using schedule_result_t = decltype(schedule(declval<Sndr>()));