Update Reference_wrapper-solution.md

Add cpp to the code block for color highlighting

Author: db-tech

c++11/Reference_wrapper-solution.md

@@ -1,6 +1,6 @@
 # Reference wrapper
 
-```
+```cpp
 #include <iostream>
 #include <vector>
 #include <numeric>
@@ -19,4 +19,4 @@ int main() {
 ```
 
 ## Links
-- [Cpp Reference](https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper)
+- [Cpp Reference](https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper)