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.

開啟 Nullish coalescing operator '??' 課程，並前往 issue 中連結的 Comparison with || 區段。檢查以「Historically, the OR || operator was there first,」開頭的句子，選擇文法正確的替換文字，並確認課程顯示修訂後的措辭。