Fixed val counter AtomicInteger in shared state example (#2063)

Fixes #2062
diff --git a/docs/shared-mutable-state-and-concurrency.md b/docs/shared-mutable-state-and-concurrency.md
index 1a3c406..96f6fdf 100644
--- a/docs/shared-mutable-state-and-concurrency.md
+++ b/docs/shared-mutable-state-and-concurrency.md
@@ -187,7 +187,7 @@
 }
 
 //sampleStart
-var counter = AtomicInteger()
+val counter = AtomicInteger()
 
 fun main() = runBlocking {
     withContext(Dispatchers.Default) {