fix typo in samples

Author: suomesta

reference/flat_map/flat_map/emplace.md

@@ -88,7 +88,7 @@ struct Point {
 
 int main()
 {
-  stdx::flat_map<std::string, Point> fm;
+  std::flat_map<std::string, Point> fm;
 
   fm.emplace(std::piecewise_construct,
              std::forward_as_tuple(3, 'C'), // キーの型std::stringのコンストラクタ引数を渡す

reference/flat_map/flat_map/empty.md

@@ -31,7 +31,7 @@
 
 int main ()
 {
-  stdx::flat_map<int, char> fm;
+  std::flat_map<int, char> fm;
 
   std::cout << fm.empty() << std::endl;
 

reference/flat_map/flat_map/op_compare_3way.md

@@ -50,13 +50,13 @@ return lexicographical_compare_three_way(
 
 int main()
 {
-  stdx::flat_map<int, char> fm1 = {
+  std::flat_map<int, char> fm1 = {
     {3, 'a'},
     {1, 'b'},
     {4, 'c'}
   };
 
-  stdx::flat_map<int, char> fm2 = {
+  std::flat_map<int, char> fm2 = {
     {3, 'a'},
     {1, 'b'},
   };