[stopcallback] Fix indentation (#4040)

Author: JohelEGP

source/threads.tex

@@ -850,10 +850,10 @@
 
     // \ref{stopcallback.cons}, constructors and destructor
     template<class C>
-    explicit stop_callback(const stop_token& st, C&& cb)
+      explicit stop_callback(const stop_token& st, C&& cb)
         noexcept(is_nothrow_constructible_v<Callback, C>);
     template<class C>
-    explicit stop_callback(stop_token&& st, C&& cb)
+      explicit stop_callback(stop_token&& st, C&& cb)
         noexcept(is_nothrow_constructible_v<Callback, C>);
     ~stop_callback();
 
@@ -867,7 +867,7 @@
   };
 
   template<class Callback>
-  stop_callback(stop_token, Callback) -> stop_callback<Callback>;
+    stop_callback(stop_token, Callback) -> stop_callback<Callback>;
 }
 \end{codeblock}