Update README.md

Author: rcalago

kotlin-math-2/README.md

@@ -3,3 +3,4 @@
 - [Find Average of All Items in a List in Kotlin](https://www.baeldung.com/kotlin/list-mean-value)
 - [How to Calculate Standard Deviation in Kotlin](https://www.baeldung.com/kotlin/standard-deviation)
 - [Check if a Number Can Be Expressed as Sum of Two Prime Numbers in Kotlin](https://www.baeldung.com/kotlin/number-sum-two-primes-test)
+- [Check if a Number Is Prime in Kotlin](https://www.baeldung.com/kotlin/primality-test)