Update DIRECTORY.md [skip actions]

Author: vil02

DIRECTORY.md

@@ -19,6 +19,7 @@
   * Bit Manipulation
     * [Counting Bits](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/counting_bits.rs)
     * [Highest Set Bit](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/highest_set_bit.rs)
+    * [N Bits Gray Code](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/n_bits_gray_code.rs)
     * [Sum Of Two Integers](https://github.com/TheAlgorithms/Rust/blob/master/src/bit_manipulation/sum_of_two_integers.rs)
   * Ciphers
     * [Aes](https://github.com/TheAlgorithms/Rust/blob/master/src/ciphers/aes.rs)