[exec.on] Add \tcode for true

Author: Andreas-Krug

source/exec.tex

@@ -3130,7 +3130,7 @@
 \pnum
 The name \tcode{on} denotes a pipeable sender adaptor object.
 For subexpressions \tcode{sch} and \tcode{sndr},
-\tcode{on(sch, sndr)} is ill-formed if any of the following is true:
+\tcode{on(sch, sndr)} is ill-formed if any of the following is \tcode{true}:
 \begin{itemize}
 \item
 \tcode{decltype((sch))} does not satisfy \libconcept{scheduler}, or