Fix React dependency to enable XCode 12 build

Author: harunsmrkovic

react-native-app-auth.podspec

@@ -13,6 +13,6 @@ Pod::Spec.new do |s|
   s.source       = { :git => 'https://github.com/FormidableLabs/react-native-app-auth.git', :tag => "v#{s.version}" }
   s.source_files  = 'ios/**/*.{h,m}'
   s.requires_arc = true
-  s.dependency 'React'
+  s.dependency 'React-Core'
   s.dependency 'AppAuth', '1.4.0'
 end