Add language specifier to code fence in TURBO_NAVIGATION_TEST_ISSUE.md

Added 'tree' as the language identifier for the fenced code block showing
the file structure, changing from generic ``` to ```tree for better
syntax highlighting and clarity.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

TURBO_NAVIGATION_TEST_ISSUE.md

@@ -143,7 +143,7 @@ npm run test:e2e -- --grep "Turbo navigation"
 
 ## Files to Create/Modify
 
-```
+```tree
 spec/dummy/spec/playwright/
   ├── turbo_navigation.spec.js          (new)
   └── support/