updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -38,6 +38,9 @@
 ## Math
   * [Fastexponent](https://github.com/TheAlgorithms/Go/blob/master/math/fastExponent.go)
   * [Greatest Common Divisor](https://github.com/TheAlgorithms/Go/blob/master/math/greatest_common_divisor.go)
+  * Prime Check
+    * [Prime Check](https://github.com/TheAlgorithms/Go/blob/master/math/prime_check/prime_check.go)
+    * [Prime Check Test](https://github.com/TheAlgorithms/Go/blob/master/math/prime_check/prime_check_test.go)
   * [Sieve](https://github.com/TheAlgorithms/Go/blob/master/math/Sieve.go)
 
 ## Other