fixup: use \tcode in code comment

Co-authored-by: Jan Schultke <[email protected]>

Author: dietmarkuehl

source/exec.tex

@@ -6289,7 +6289,7 @@
     allocator_type    @\exposidnc{alloc}@;  // \expos
     stop_source_type  @\exposidnc{source}@; // \expos
     stop_token_type   @\exposidnc{token}@;  // \expos
-    optional<T>       @\exposidnc{result}@; // \expos; present only if is_void_v<T> is false;
+    optional<T>       @\exposidnc{result}@; // \expos; present only if \tcode{is_void_v<T>} is \tcode{false};
     @\exposid{error-variant}@      @\exposidnc{errors}@; // \expos
   };
 }