added link for nth_element

Author: rotarymars

reference/algorithm/nth_element.md

@@ -86,6 +86,12 @@ int main()
 }
 ```
 * std::nth_element[color ff0000]
+* std::vector[link /reference/vector/vector.md]
+* v.begin[link /reference/vector/vector/begin.md]
+* v.end[link /reference/vector/vector/end.md]
+* std::for_each[link /reference/algorithm/for_each.md]
+* std::cout[link /reference/iostream/cout.md]
+* std::endl[link /reference/ostream/endl.md]
 
 ### 出力例
 ```