[time.duration.general] Use code font for duration

Author: hewillk

source/time.tex

@@ -1279,7 +1279,7 @@
 those thrown by the indicated operations on their representations.
 
 \pnum
-The defaulted copy constructor of duration shall be a
+The defaulted copy constructor of \tcode{duration} shall be a
 constexpr function if and only if the required initialization
 of the member \tcode{rep_} for copy and move, respectively, would
 be constexpr-suitable\iref{dcl.constexpr}.