fix: code diff display in dropdown-menu documentation

[terrence-ou]

Hi,

While reading the documentation for the Dropdown Menu component, I noticed some syntax highlighting issues in the changelog section that might be a bit confusing :

The code blocks in the section are visually inconsistent. One block is missing the “+” sign to emphasize the added code, another block uses tsx syntax highlighting, which doesn’t align with the purpose of the section that suggests developers to add additional tailwind styles.

To address these issues, I made the following changes:

• Updated both code blocks in the section with diff syntax highlighting for code display consistency.
• Added “+” signs to emphasize changes.
• Replaced the cva reference in the second paragraph of the 2024-10-25... changelog with cn. The cva function seems not used in the associated code block, but cn is.

Please refer to the following screenshots for the changes I made on the doc, with left one is before change, and the right one is after change:

Thanks for considering my PR, and please let me know if you have any feedback or additional suggestions!

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