docs: Update README.md (sveltejs#520)

* Update README.md

docs: 04-updating-arrays-and-objects, add `let obj = { foo: { bar: 'old' } };` for more clarity

* Apply suggestions from code review

---------

Co-authored-by: Rich Harris <[email protected]>

Author: matzar

content/tutorial/01-svelte/02-reactivity/04-updating-arrays-and-objects/README.md

@@ -38,8 +38,9 @@ A simple rule of thumb: the name of the updated variable must appear on the left
 
 ```js
 /// no-file
+const obj = { foo: { bar: 1 } };
 const foo = obj.foo;
-foo.bar = 'baz';
+foo.bar = 2;
 ```
 
 ...won't trigger reactivity on `obj.foo.bar`, unless you follow it up with `obj = obj`.