[exec.async.ops] Add \tcode for true (#7233)

Author: hewillk

source/exec.tex

@@ -358,7 +358,7 @@
 the function type \tcode{decltype(auto(set))(decltype((args))...)}.
 A completion signature \tcode{Sig} is associated with \tcode{c}
 if and only if
-\tcode{\exposid{MATCHING-SIG}(Sig, F)} is true\iref{exec.general}).
+\tcode{\exposid{MATCHING-SIG}(Sig, F)} is \tcode{true}\iref{exec.general}).
 Together, a sender type and an environment type \tcode{Env} determine
 the set of completion signatures of an asynchronous operation
 that results from connecting the sender with a receiver