[exec.snd.expos] Move specification of default template argument for 'Data'

Fixes NB US 212-352 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -1716,6 +1716,12 @@
 where \exposid{basic-sender} is the following exposition-only class template except as noted below.
 \end{itemdescr}
 
+\pnum
+\remarks
+The default template argument for the \tcode{Data} template parameter
+denotes an unspecified empty trivially copyable class type
+that models \libconcept{semiregular}.
+
 \begin{codeblock}
 namespace std::execution {
   template<class Tag>
@@ -1845,11 +1851,6 @@
 }
 \end{codeblock}
 
-\pnum
-The default template argument for the \tcode{Data} template parameter
-denotes an unspecified empty trivially copyable class type
-that models \libconcept{semiregular}.
-
 \pnum
 It is unspecified whether a specialization of \exposid{basic-sender}
 is an aggregate.