[exec.just] Add missing LaTeX escape for product-type construction (#7216)

Author: lewissbaker

source/exec.tex

@@ -2583,7 +2583,7 @@
 \end{itemize}
 
 Otherwise, it is expression-equivalent to
-\tcode{\exposid{make-sender}(\exposid{just-cpo}, \exposid{product-type}{ts...})}.
+\tcode{\exposid{make-sender}(\exposid{just-cpo}, \exposid{product-type}\{ts...\})}.
 
 For \tcode{just}, \tcode{just_error}, and \tcode{just_stopped},
 let \exposid{set-cpo} be