Update README.md (#32277)

Summary:
Update the showcase url to the right url.

When I did check out the repository and I was about to navigate to showcase. I found out the url was wrong.

## Changelog

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[General] [Changed] - Changed the showcase URL

Pull Request resolved: https://github.com/facebook/react-native/pull/32277

Reviewed By: lunaleaps

Differential Revision: D31206326

Pulled By: cortinico

fbshipit-source-id: a4092e9bb74d9a9cc68d7252aa131083dd38b6ca

README.md

@@ -32,7 +32,7 @@
   <span> · </span>
   <a href="https://reactnative.dev/docs/tutorial">Learn the Basics</a>
   <span> · </span>
-  <a href="https://reactnative.dev/showcase.html">Showcase</a>
+  <a href="https://reactnative.dev/showcase">Showcase</a>
   <span> · </span>
   <a href="https://reactnative.dev/docs/contributing">Contribute</a>
   <span> · </span>