update README: use onevent syntax rather than on:event

[aster-void]

From svelte 5, svelte uses onevent syntax. Therefore I updated the example from old on:event to not confuse the users.

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[aster-void]

Side note, the example code from src/routes/Chat.svelte doesn't seem to cancel form submission
https://github.com/get-convex/convex-svelte/blob/main/src/routes/Chat.svelte