[time] Fix namespace for definitions in std::chrono

The previous commit ad767d3 accidentally added the wrong namespace "std" instead of "std::chrono".

Author: tkoeppe

source/utilities.tex

@@ -17522,7 +17522,7 @@
 
 \indexlibrary{\idxcode{duration}}%
 \begin{codeblock}
-namespace std {
+namespace std::chrono {
   template <class Rep, class Period = ratio<1>>
     class duration {
     public:
@@ -18284,7 +18284,7 @@
 
 \indexlibrary{\idxcode{time_point}}%
 \begin{codeblock}
-namespace std {
+namespace std::chrono {
   template <class Clock, class Duration = typename Clock::duration>
     class time_point {
     public:
@@ -18638,7 +18638,7 @@
 realtime clock.
 
 \begin{codeblock}
-namespace std {
+namespace std::chrono {
   class system_clock {
   public:
     using rep        = @\seebelow@;
@@ -18707,7 +18707,7 @@
 a steady rate relative to real time. That is, the clock may not be adjusted.
 
 \begin{codeblock}
-namespace std {
+namespace std::chrono {
   class steady_clock {
   public:
     using rep        = @\unspec@;
@@ -18730,7 +18730,7 @@
 \tcode{system_clock} or \tcode{steady_clock}.
 
 \begin{codeblock}
-namespace std {
+namespace std::chrono {
   class high_resolution_clock {
   public:
     using rep        = @\unspec@;