[cpp.predefined] Add \tcode for integer literal (#8750)

Author: Halalaluyafail3

source/preprocessor.tex

@@ -2446,7 +2446,7 @@
 \item
 \indextext{__stdcpp_threads__@\mname{STDCPP_THREADS}}%
 \mname{STDCPP_THREADS}\\
-Defined, and has the value integer literal 1, if and only if a program
+Defined, and has the value integer literal \tcode{1}, if and only if a program
 can have more than one thread of execution\iref{intro.multithread}.
 
 \end{description}