Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/ecs_task/exercise.md

@@ -0,0 +1,9 @@
+## AWS Containers - Run Tasks
+
+Note: this costs money
+
+### Objectives
+
+Create a task in ECS to launch in Fargate.
+
+The task itself can be a sample app.