[thread.condition] Move index entry for library name from main index to library

index.

Author: zygoloid

source/threads.tex

@@ -2722,7 +2722,7 @@
 }
 \end{codeblock}
 
-\indextext{\idxcode{notify_all_at_thread_exit}}%
+\indexlibrary{\idxcode{notify_all_at_thread_exit}}%
 \begin{itemdecl}
 void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);
 \end{itemdecl}