[optional.ctor] Add missing \tcode around 'true' (#4534)

Author: cjdb

source/utilities.tex

@@ -2748,7 +2748,7 @@
 \pnum
 \constraints
 \begin{itemize}
-\item \tcode{is_constructible_v<T, U>} is true,
+\item \tcode{is_constructible_v<T, U>} is \tcode{true},
 \item \tcode{is_constructible_v<T, optional<U>\&>} is \tcode{false},
 \item \tcode{is_constructible_v<T, optional<U>\&\&>} is \tcode{false},
 \item \tcode{is_constructible_v<T, const optional<U>\&>} is \tcode{false},