Fix deprecated mode attribute in README generator prompt (#41600)

Co-authored-by: Sharra-writes <[email protected]>

Author: daisukedd

content/copilot/tutorials/customization-library/prompt-files/create-readme.md

@@ -23,7 +23,7 @@ This prompt file creates professional, comprehensive README files by analyzing y
 
 ```text copy
 ---
-mode: 'agent'
+agent: 'agent'
 description: 'Create a comprehensive README.md file for the project'
 ---