Fix .gitignore for packages/react-native/sdks/downloads (#39963)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39963

Changelog: [Internal] - Fix typo in .gitignore for downloaded sdks

ignore-github-export-checks

Reviewed By: huntie

Differential Revision: D50018994

fbshipit-source-id: 3a92bb6cfee214d7265639a8dd19c6367ff0c7cd