docs(readme): fix minimum iOS version in requirements section (facebook#34134)

Summary:
`[email protected]` bumped minimum iOS version to 12.4. This change aligns the readme requirements section.

Changelog:
[General] [Changed] - Doc: fix minimum iOS version in requirements section

Pull Request resolved: facebook#34134

Reviewed By: cortinico

Differential Revision: D37656176

Pulled By: philIip

fbshipit-source-id: 017147140340b4ccb6d8c4ff2eb50220fbdba22c

Author: monholm

README.md

@@ -67,7 +67,7 @@ React Native is developed and supported by many companies and individual core co
 
 ## 📋 Requirements
 
-React Native apps may target iOS 11.0 and Android 5.0 (API 21) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like [Expo](https://expo.io) can be used to work around this.
+React Native apps may target iOS 12.4 and Android 5.0 (API 21) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like [Expo](https://expo.io) can be used to work around this.
 
 ## 🎉 Building your first React Native app