Make Minor Improvement to Grammar on Nullish coalescing operator '??' Lesson

In the lesson titled Nullish coalescing operator '??', in the Comparison with II section, located here, there is a minor mistake in the grammar.

"Historically, the OR || operator was there first. It exists since the beginning of JavaScript, so developers were using it for such purposes for a long time."

Should become something like

"Historically, the OR || operator was there first. It's existed since the beginning of JavaScript, so developers were using it for such purposes for a long time."

or

"Historically, the OR || operator was there first. It's been there since the beginning of JavaScript, so developers were using it for such purposes for a long time."

Thank you for these excellent lessons. This is my first attempt to contribute something to a project via Github, I hope it's formatted properly.

Mở bài học về toán tử Nullish coalescing '??' và đi đến phần Comparison with || được liên kết trong issue. Xem lại câu bắt đầu bằng “Historically, the OR || operator was there first,”, chọn một cách thay thế đúng ngữ pháp và xác nhận rằng bài học hiển thị cách diễn đạt đã sửa.