Update shared-mutable-state-and-concurrency.md

Main func should return Unit, otherwise it won't run

Author: AlexBlokh

docs/shared-mutable-state-and-concurrency.md

@@ -501,7 +501,7 @@ fun CoroutineScope.counterActor() = actor<CounterMsg> {
 }
 
 //sampleStart
-fun main() = runBlocking {
+fun main() = runBlocking<Unit> {
     val counter = counterActor() // create the actor
     withContext(Dispatchers.Default) {
         massiveRun {