fix: bug in reactive classes tutorial (app b)

[M0ST4FA]

Issue #443

Make sure the embiggen button never embiggens rectangle dimensions beyond MAX_SIZE.

Note: the same bug exists in "app a" but I don't understand why there are two apps in the first place, so I chose one randomly. Also, I checked the documentation repo and I couldn't find the tutorials in the directory tree, so sorry if the PR is in the wrong place.

[vercel]

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[Conduitry]

There are two apps because one is what the user is initially presented with and the other is the solved version.

This isn't the right fix, though. We handle MAX_SIZE in a later tutorial - https://svelte.dev/tutorial/svelte/getters-and-setters

I think, if anything, perhaps MAX_SIZE should be removed from the earlier tutorial.