Fix missing / in podspec (#624)

Author: CedricGuillemet

Modules/@babylonjs/react-native-iosandroid/react-native-babylon.podspec

@@ -15,7 +15,7 @@ Pod::Spec.new do |s|
 
   s.source_files = "ios/*.{h,m,mm}"
   s.requires_arc = true
-  s.xcconfig     = { 'USER_HEADER_SEARCH_PATHS' => '$(inherited) ${PODS_TARGET_SRCROOT}/shared ${PODS_TARGET_SRCROOT}/..react-native/shared' }
+  s.xcconfig     = { 'USER_HEADER_SEARCH_PATHS' => '$(inherited) ${PODS_TARGET_SRCROOT}/shared ${PODS_TARGET_SRCROOT}/../react-native/shared' }
 
   s.libraries = 'astc-encoder',
                 'etc1',