updated README for part1/reactivity/updating-arrays-and-objects

[two048]

Part 1/Reactivity/Updating arrays and objects has an example

/// no-file
const obj = { foo: { bar: 1 } };
const foo = obj.foo;
foo.bar = 2;

Stating that this "...won't trigger reactivity on obj.foo.bar, unless you follow it up with obj = obj."

Which is true. But either way when an ignorant beginner like me reaches this example, they would struggle with it, as using obj = obj would present them with an error as such /src/lib/App.svelte:13:2 You are assigning to a const. So I changed it to:

/// no-file
let obj = { foo: { bar: 1 } };
let foo = obj.foo;
foo.bar = 2;

If this is I request this PR be merged.

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