[time.traits.specializations] Fix index entry for common_type specialization (#7174)

Author: Eelis

source/time.tex

@@ -1183,7 +1183,7 @@
 floating-point durations can have round-off errors.
 \end{note}
 
-\indexlibraryglobal{common_type}%
+\indexlibrarymember{common_type}{duration}%
 \begin{itemdecl}
 template<class Clock, class Duration1, class Duration2>
   struct common_type<chrono::time_point<Clock, Duration1>, chrono::time_point<Clock, Duration2>> {