docs: fix using reserved word in example

[HarryAllen1]

Currently, in the Svelte 5 Migration guide, under the $props migration, the example uses class as a variable name which it passes down to a button. However, the class variable is not declared and cannot be declared, so this PR changes the variable to access the one which actually exists.

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Latest commit: cf7aeca

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[dummdidumm]

thank you!