Update c++14-features.md

Author: akorach

c++14-features.md

@@ -132,7 +132,7 @@ written out [String]
 - [x] std::exchange
 ```Replaces the value of obj with new_value and returns the old value of obj. Not to be confused with std::swap, where swap takes in two parameters and swaps the values, and returns nothing.```
  * Potentially helpful when writing move assignment or constructors.
-example at the bottom of page: https://docs.w3cub.com/cpp/utility/exchange/
+example at the bottom of page: https://docs.w3cub.com/cpp/utility/exchange/ or https://en.cppreference.com/w/cpp/utility/exchange
 - [x] “Fixing constexpr member functions without const”
 
 If C++14 `constexpr` member functions are not declared `const`, they are `mutable` and may modify the objects of which they are members, as long as that object's lifetime started during the execution of the `constexpr` method.