fix: Xcode 12 compatibility

Author: mrbrentkelly

react-native-pdf.podspec

@@ -14,5 +14,5 @@ Pod::Spec.new do |s|
   s.requires_arc   = true
   s.platform       = :ios, '8.0'
   s.source_files   = 'ios/**/*.{h,m}'
-  s.dependency     'React'
+  s.dependency     'React-Core'
 end