[stack.syn] Show the formatter specialization in the synopsis

Author: frederick-vs-ja

source/containers.tex

@@ -13734,6 +13734,10 @@
     void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y)));
   template<class T, class Container, class Alloc>
     struct uses_allocator<stack<T, Container>, Alloc>;
+
+  // \ref{container.adaptors.format}, formatter specialization for \tcode{stack}
+  template<class charT, class T, @\libconcept{formattable}@<charT> Container>
+    struct formatter<stack<T, Container>, charT>;
 }
 \end{codeblock}