[tuple.cnstr] Add missing \tcode (#1796)

Author: Quuxplusone

source/utilities.tex

@@ -1254,7 +1254,7 @@
 constexpr functions.
 
 \pnum
-The destructor of tuple shall be a trivial destructor if
+The destructor of \tcode{tuple} shall be a trivial destructor if
 \tcode{(is_trivially_destructible_v<Types> \&\& ...)}
 is \tcode{true}.