[fix] a typo

STL/STL.md

@@ -62,7 +62,7 @@ int main()
 {
     std::array<int, 5> myarray = {2, 16, 77,34, 50};
     std::cout << "myarray contains:";
-    for(auto it = myarray.begin(); it != myarray.end(); ++i)
+    for(auto it = myarray.begin(); it != myarray.end(); ++it)
         std::cout << ' ' << *it;
     std::cout << '\n';