Update covid-19 topic desc

[ynshung]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

The pandemic is not really ongoing anymore so changed it to be more generic.

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[Copilot]

Pull Request Overview

This pull request updates the COVID-19 topic description to reflect that the pandemic is in the past rather than ongoing.

• Updated the phrasing to describe COVID-19 as having caused a "worldwide pandemic beginning in December 2019".
• Revised content in topics/covid-19/index.md to better represent the historical context of the disease.