[exec.par.scheduler] Move class definition from synopsis

Fixes NB US 204-321 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -724,7 +724,7 @@
 
 namespace std::execution {
   // \ref{exec.par.scheduler}, parallel scheduler
-  class @\libglobal{parallel_scheduler}@ { @\unspec@ };
+  class @\libglobal{parallel_scheduler}@;
   parallel_scheduler get_parallel_scheduler();
 }
 
@@ -8279,6 +8279,14 @@
 
 \rSec1[exec.par.scheduler]{Parallel scheduler}
 
+\begin{codeblock}
+namespace std::execution {
+  class @\libglobal{parallel_scheduler}@ {
+    @\unspec@
+  };
+}
+\end{codeblock}
+
 \pnum
 \tcode{parallel_scheduler} models \libconcept{scheduler}.