Add hyperlink for Good First Issues header (#29103)

## Summary

The "Good First Issues" header in the README was missing a hyperlink
where the other similar headlines had one.

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## How did you test this change?

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Author: NickeSteenen

README.md

@@ -69,7 +69,7 @@ Facebook has adopted a Code of Conduct that we expect project participants to ad
 
 Read our [contributing guide](https://legacy.reactjs.org/docs/how-to-contribute.html) to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React.
 
-### Good First Issues
+### [Good First Issues](https://github.com/facebook/react/labels/good%20first%20issue)
 
 To help you get your feet wet and get you familiar with our contribution process, we have a list of [good first issues](https://github.com/facebook/react/labels/good%20first%20issue) that contain bugs that have a relatively limited scope. This is a great place to get started.