Update tutorial command to use model 'claude-sonnet-4.5'

Author: dervla-instil

README.md

@@ -14,7 +14,7 @@ An interactive, hands-on tutorial for learning GitHub Copilot CLI — built enti
 git clone git@github.com:instil/github-copilot-tutorial.git
 cd github-copilot-tutorial
 npm install --prefix exercises/sample-app
-copilot -i "start the tutorial" --model "claude-sonnet-4.6"
+copilot -p "start the tutorial" --model "claude-sonnet-4.5"
 ```
 
 This command automatically starts the tutorial and shows you the welcome screen.