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.

打开 Nullish coalescing operator '??' 课程，然后转到 issue 中链接的 Comparison with || 部分。检查以“Historically, the OR || operator was there first,”开头的句子，选择一个语法正确的替换文本，并确认课程显示修改后的措辞。