Fix wrong comment in basic-reactivity.md

[JohnNiang]

Description of Problem

Wrong comment obj.value as below:

// it will update `obj.value`
count.value++
console.log(count.value) // 2
console.log(obj.count) // 2

Proposed Solution

- // it will update `obj.value`
+ // it will update `obj.count`

Additional Information

[NataliaTepluhina]

@JohnNiang thank you for the fix!