Fixup: escape ampersand

Co-authored-by: A. Jiang <[email protected]>

Author: eisenwave

source/utilities.tex

@@ -4533,7 +4533,7 @@
 is a reference to \tcode{*\exposid{val}}.
 
 \pnum
-Each type \tcode{optional<T&>} is a trivially copyable class\iref{class.prop}.
+Each type \tcode{optional<T\&>} is a trivially copyable class\iref{class.prop}.
 
 \rSec3[optional.ref.ctor]{Constructors}