Fix example-flow-14 docs to show expected exception in text

Fixes #1582

Author: elizarov

docs/flow.md

@@ -708,13 +708,15 @@ fun main() = runBlocking<Unit> {
 
 This code produces the following exception:
 
-<!--- TEST EXCEPTION
+```text
 Exception in thread "main" java.lang.IllegalStateException: Flow invariant is violated:
 		Flow was collected in [CoroutineId(1), "coroutine#1":BlockingCoroutine{Active}@5511c7f8, BlockingEventLoop@2eac3323],
 		but emission happened in [CoroutineId(1), "coroutine#1":DispatchedCoroutine{Active}@2dae0000, DefaultDispatcher].
 		Please refer to 'flow' documentation or use 'flowOn' instead
 	at ...
--->
+``` 
+
+<!--- TEST EXCEPTION -->
 
 > Note that we had to use a fully qualified name of the [kotlinx.coroutines.withContext][withContext] function in this example to 
 demonstrate this exception. A short name of `withContext` would have resolved to a special stub function that