[stoptoken.concepts] Use \tcode only for code.

Author: Eelis

source/threads.tex

@@ -599,7 +599,7 @@
 An object of type \tcode{SCB} has
 an associated callback function of type \tcode{CallbackFn}.
 Let \tcode{scb} be an object of type \tcode{SCB} and
-let \tcode{callback_fn} denote \tcode{scb's} associated callback function.
+let \tcode{callback_fn} denote \tcode{scb}'s associated callback function.
 Direct-non-list-initializing \tcode{scb} from
 arguments \tcode{t} and \tcode{init}
 shall execute a \defnadj{stoppable callback}{registration} as follows: