[flat.map.syn] Add missing "namespace std {"

Author: tkoeppe

source/containers.tex

@@ -13240,6 +13240,7 @@
 #include <compare>              // see \ref{compare.syn}
 #include <initializer_list>     // see \ref{initializer.list.syn}
 
+namespace std {
   // \ref{flat.map}, class template \tcode{flat_map}
   template<class Key, class T, class Compare = less<Key>,
            class KeyContainer = vector<Key>, class MappedContainer = vector<T>>