Improve alt-text for serverless economics image

Updated alt-text for image to clarify its content.

Author: JamesJBarnett

learn-pr/wwl-azure/describe-azure-compute-networking-services/includes/6-functions.md

@@ -14,7 +14,7 @@ Functions scale automatically based on demand, so they may be a good choice when
 
 Azure Functions runs your code when it's triggered and automatically deallocates resources when the function finishes. In this model, Azure charges you only for the CPU time used while your function runs.
 
-:::image type="content" source="../media/functions-serverless-economics.png" alt-text="Serverless runs scale with events and bill only while running.":::
+:::image type="content" source="../media/functions-serverless-economics.png" alt-text="Diagram showing serverless runs scale with events and bill only while running.":::
 
 Functions can be either stateless or stateful. When they're stateless (the default), they behave as if they restart every time they respond to an event. When they're stateful (called Durable Functions), the runtime passes a context through the function to track prior activity.