fix: fix hint to match animation used in solution

[chhotii-alex]

The code (both app-a and app-b) references the fly transition, but the hint mentions a different transition (fade). Probably less confusing for a newbie if the hint does not have this extraneous discrepancy. Fixed hint in description to match problem code.

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[benmccann]

This is the old tutorial. The new tutorial hosted on https://learn.svelte.dev is here: https://github.com/sveltejs/learn.svelte.dev/tree/main/content/tutorial/02-advanced-svelte/02-transitions/08-key-blocks