[exec.schedule.from] Add missing \exposid to child-type

Author: jwakely

source/exec.tex

@@ -3017,7 +3017,7 @@
 \pnum
 Let \tcode{Sigs} be
 a pack of the arguments to the \tcode{completion_signatures} specialization
-named by \tcode{completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>}.
+named by \tcode{completion_signatures_of_t<@\exposid{child-type}<Sndr>, env_of_t<Rcvr>>}.
 Let \exposid{as-tuple} be an alias template
 that transforms a completion signature \tcode{Tag(Args...)} into
 the tuple specialization \tcode{\exposid{decayed-tuple}<Tag, Args...>}.