Update prompt file to use 'agent' key format

Author: rmoreirao

content/copilot/tutorials/customization-library/prompt-files/your-first-prompt-file.md

@@ -32,7 +32,7 @@ Start with this simple prompt file that helps you write clear, well-documented c
 
 ```text copy
 ---
-mode: 'agent'
+agent: 'agent'
 description: 'Generate a clear code explanation with examples'
 ---