docs(bash-cli): add txt language hint to example output code block

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: appleboy

bash-cli/README.md

@@ -56,7 +56,7 @@ bash main.sh
 
 ## Example Output
 
-```
+```txt
 To sign in, open the following URL in a browser:
 
   https://auth.example.com/device