fix(example): correct swift package manager configuration in pubspec

Author: SifAa

flutter_readium/example/pubspec.yaml

@@ -35,7 +35,8 @@ dev_dependencies:
 
 flutter:
   uses-material-design: true
-  disable-swift-package-manager: true
+  config:
+    enable-swift-package-manager: false
   assets:
     - assets/pubs/
     - assets/webManifestList.json