Rename exercises dir

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

topics/aws/exercises/url_function/exercise.md

@@ -0,0 +1,7 @@
+## URL Function
+
+Create a basic AWS Lambda function that will be triggered when you enter a URL in the browser
+
+## Solution
+
+Click [here to view to solution](solution.md)