[time.duration.nonmember] Fix index entries for operators (#7177)

Author: Eelis

source/time.tex

@@ -1601,7 +1601,7 @@
 In the function descriptions that follow, unless stated otherwise,
 let \tcode{CD} represent the return type of the function.
 
-\indexlibraryglobal{common_type}%
+\indexlibrarymember{operator+}{duration}%
 \begin{itemdecl}
 template<class Rep1, class Period1, class Rep2, class Period2>
   constexpr common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>
@@ -1614,11 +1614,11 @@
 \tcode{CD(CD(lhs).count() + CD(rhs).count())}.
 \end{itemdescr}
 
-\indexlibraryglobal{common_type}%
+\indexlibrarymember{operator-}{duration}%
 \begin{itemdecl}
 template<class Rep1, class Period1, class Rep2, class Period2>
   constexpr common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>
-  operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
+    operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}