Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-concurrency-3/README.md

@@ -6,3 +6,4 @@ This module contains articles about concurrency in Kotlin.
 - [Comparing wait(), sleep(), and delay() in Kotlin](https://www.baeldung.com/kotlin/wait-sleep-delay-difference)
 - [Difference Between collect() and collectLatest() in Kotlin Flows](https://www.baeldung.com/kotlin/flow-collect-vs-collectlatest)
 - [Difference Between Value and Emit in MutableStateFlow Kotlin](https://www.baeldung.com/kotlin/mutablestateflow-value-vs-emit)
+- [Difference Between single() and first() in Kotlin Flows](https://www.baeldung.com/kotlin/flows-single-vs-first)