fix(event_loop):make outline&title consistency.

[Cecil0o0]

Summary

The article, which describes the event loop of javascript language, from project mdn/content, was been titled The event loop. Meanwhile, the relative outline included by sidebar is not.

Problem

The problem is that it could be confused by in-consistency between them above for our readers.

Solution

Before I create a pr for mdn/yari, I created a pr for mdn/content project, this one mdn/content#35836.

I got two helpful tips.

• One is that Concurrency is removed on purpose Remove “concurrency” from Web/JavaScript/EventLoop title & text content#12354.
• Another is that outline included by sidebar was provided by this project~

This pr would like to solve the problem, want to improve readability of article by making outline and title consistency for our readers.

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Screenshots

Before

After

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How did you test this change?

compare by two screenshots above and recognize differences.

[Cecil0o0]

@LeoMcA Hi, could you please have a look at this pr? ^_^ I'm glad to have your reaction or response~