docs: add the link of the algorithm to DIRECTORY.md

Author: ThiagoFBastos

DIRECTORY.md

@@ -209,6 +209,7 @@
     * [Strongly Connected Components](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/strongly_connected_components.rs)
     * [Tarjans Strongly Connected Components](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/tarjans_ssc.rs)
     * [Topological Sort](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/topological_sort.rs)
+    * [Tree Center](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/tree_center.rs)
     * [Two Satisfiability](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/two_satisfiability.rs)
   * Greedy
     * [Minimum Coin Change](https://github.com/TheAlgorithms/Rust/blob/master/src/greedy/minimum_coin_changes.rs)