Update podspec to allow auto-linking for RN

facebook/react-native#25553 (comment)

Referencing from this comment, the fix is to update the:
`s.dependency 'React'` to `s.dependency 'React-Core` to make everything auto-link properly in iOS. 

All react-native libraries with native iOS code should be doing this to keep up to date with the requirements in react-native 60 and up.

Author: suark

RNSha256.podspec

@@ -17,5 +17,5 @@ Pod::Spec.new do |s|
   s.source_files = "ios/**/*.{h,m,swift}"
   s.preserve_paths  = "**/*.js"
 
-  s.dependency 'React'
-end
+  s.dependency 'React-Core'
+end