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.

Nullish coalescing operator '??' のレッスンを開き、issue でリンクされている Comparison with || セクションに移動します。“Historically, the OR || operator was there first,” で始まる文を確認し、文法的に正しい置き換えを選び、レッスンに修正後の文言が表示されることを確認します。