docs: Update README.md

[matzar]

Docs: 04-updating-arrays-and-objects

Ensured clarity in the example code by defining the obj object. This ensures that readers understand the context and structure of the object being modified in the reactivity example.

Because it is possible to trigger reactivity by referencing foo.bar in the template instead of obj.foo.bar, the following will trigger reactivity:

<script>
    let obj = { foo: { bar: 'old' } };
    const foo = obj.foo;
    
    function updateFooBar() {
        foo.bar = 'baz';
    }
</script>

<button on:click={updateFooBar}>Update foobar</button>
<!-- referencing the variable with `foo.bar` - triggers reactivity -->
<p>baz is: {foo.bar}</p>

However, the next example will not, and will require obj = obj reassignment:

<script>
    let obj = { foo: { bar: 'old' } };
    const foo = obj.foo;
    
    function updateFooBar() {
        foo.bar = 'baz';
        // uncomment below to trigger reactivity
        // obj = obj 
    }
</script>

<button on:click={updateFooBar}>Update foobar</button>
<!-- referencing the variable with `obj.foo.bar` - does **not** trigger reactivity -->
<p>baz is: {obj.foo.bar}</p>

In this case, {obj.foo.bar} does not trigger reactivity when foo.bar is updated, unless obj is reassigned (with obj = obj).

By including the definition of the obj object, we aim to elucidate the structure and context in which the reactivity example operates, ensuring a clearer understanding for the reader.

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[Rich-Harris]

thanks!