Update DIRECTORY.md [skip actions]

Author: vil02

DIRECTORY.md

@@ -317,6 +317,7 @@
     * [Burrows Wheeler Transform](https://github.com/TheAlgorithms/Rust/blob/master/src/string/burrows_wheeler_transform.rs)
     * [Duval Algorithm](https://github.com/TheAlgorithms/Rust/blob/master/src/string/duval_algorithm.rs)
     * [Hamming Distance](https://github.com/TheAlgorithms/Rust/blob/master/src/string/hamming_distance.rs)
+    * [Isogram](https://github.com/TheAlgorithms/Rust/blob/master/src/string/isogram.rs)
     * [Isomorphism](https://github.com/TheAlgorithms/Rust/blob/master/src/string/isomorphism.rs)
     * [Jaro Winkler Distance](https://github.com/TheAlgorithms/Rust/blob/master/src/string/jaro_winkler_distance.rs)
     * [Knuth Morris Pratt](https://github.com/TheAlgorithms/Rust/blob/master/src/string/knuth_morris_pratt.rs)