ES6 Modules lesson: Remove variable's name in export default examples and edit description

[StepQuest]

Because

Below the heading Default exports, ware fixed mistakes in the given examples of using export default, where the variable name was not removed, which led to a ReferenceError

This PR

• Within two code blocks, export default greeting = "Hello, Odinite!"; was changed to export default "Hello, Odinite!";
• The description before of the first example was a little bit edited

Issue

Additional Information

https://discord.com/channels/505093832157691914/505093832157691916/1275465471059034196

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
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• If any lesson files are included in this PR, they follow the Layout Style Guide

[MaoShizhong]

Thanks for catching and fixing my error!