Fix React dependency to enable XCode 12 build (#347)

Author: harunsmrkovic

react-native-document-picker.podspec

@@ -12,5 +12,5 @@ Pod::Spec.new do |s|
   s.source          = { :git => "https://github.com/Elyx0/react-native-document-picker", :tag => "v#{s.version}" }
   s.source_files    = "ios/RNDocumentPicker/*.{h,m}"
   s.platform        = :ios, "9.0"
-  s.dependency        'React'
+  s.dependency        'React-Core'
 end