[exec.spawn.future] add missing \tcode

Author: ericniebler

source/exec.tex

@@ -5339,9 +5339,9 @@
 \end{codeblock}
 
 \pnum
-Let \placeholder{ssource-t} be an unspecified type
+Let \tcode{\placeholder{ssource-t}} be an unspecified type
 that models \exposconcept{stoppable-source} and
-let \tcode{ssource} be an lvalue of type \placeholder{ssource-t}.
+let \tcode{ssource} be an lvalue of type \tcode{\placeholder{ssource-t}}.
 Let \tcode{\placeholder{stoken-t}} be \tcode{decltype(ssource.get_token())}.
 Let \exposid{future-spawned-sender} be the alias template: