docs:Update 02-context.md

[simon-lund]

use const for key variable and assign it a symbol instead of empty of object.

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Preview: https://svelte-dev-git-preview-svelte-15700-svelte.vercel.app/

[Rich-Harris]

Thanks but there's no real benefit to using a symbol here, it just makes it look like you're supposed to use one when in reality any value will do; as such I think it adds confusion

[simon-lund]

I see your point, I was just confused by the key variable being an empty object. I guess someone will always be confused, so yea no need to update it. Maybe if this is a common thing to do, one could add a comment, because initially I thought this was an error in the docs as I did not expect this to work 😅