docs: change className to class in theming page

[Muneersahel]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/goetzrobin/spartan/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [x] Documentation content changes
• [ ] Other... Please describe:

Which package are you modifying?

• [ ] accordion
• [ ] alert
• [ ] alert-dialog
• [ ] aspect-ratio
• [ ] avatar
• [ ] badge
• [ ] button
• [ ] calendar
• [ ] card
• [ ] checkbox
• [ ] collapsible
• [ ] combobox
• [ ] command
• [ ] context-menu
• [ ] data-table
• [ ] date-picker
• [ ] dialog
• [ ] dropdown-menu
• [ ] hover-card
• [ ] icon
• [ ] input
• [ ] label
• [ ] menubar
• [ ] navigation-menu
• [ ] pagination
• [ ] popover
• [ ] progress
• [ ] radio-group
• [ ] scroll-area
• [ ] select
• [ ] separator
• [ ] sheet
• [ ] skeleton
• [ ] slider
• [ ] sonner
• [ ] spinner
• [ ] switch
• [ ] table
• [ ] tabs
• [ ] textarea
• [ ] toast
• [ ] toggle
• [ ] tooltip
• [ ] typography

What is the current behavior?

Angular uses class instead of className, and the sample code in theming page used className instead

Closes #

What is the new behavior?

The sample code in theming page uses class instead of className

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information