Fix: update sidebar background-color variable

[holtergram]

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

changed bg-sidebarto bg-sidebar-background according to the css variable naming.

[zernonia]

The current bg-sidebar is correct

https://github.com/shadcn-ui/ui/blob/main/apps/www/registry/default/ui/sidebar.tsx#L144C8-L144C99