Fix doc

Author: 巳月

core/kotlinx-coroutines-core/test/guide/example-exceptions-04.kt

@@ -8,7 +8,7 @@ package kotlinx.coroutines.guide.exceptions04
 import kotlinx.coroutines.*
 
 fun main() = runBlocking {
-//sampleEnd
+//sampleStart
     val handler = CoroutineExceptionHandler { _, exception -> 
         println("Caught $exception") 
     }

docs/exception-handling.md

@@ -206,7 +206,7 @@ when its child completes with exception despite the installed handler.
 import kotlinx.coroutines.*
 
 fun main() = runBlocking {
-//sampleEnd
+//sampleStart
     val handler = CoroutineExceptionHandler { _, exception -> 
         println("Caught $exception") 
     }