[thread.thread.id] Use struct instead of class for the formatter<thread::id, charT> specialization added in P2693R1

Author: burblebee

source/threads.tex

@@ -1305,7 +1305,7 @@
 
 \indexlibrary{\idxcode{formatter}!specializations!\idxcode{thread::id}}%
 \begin{itemdecl}
-template<class charT> class formatter<thread::id, charT>;
+template<class charT> struct formatter<thread::id, charT>;
 \end{itemdecl}
 
 \begin{itemdescr}