[file_selector_web] Fix typo in pubspec. (flutter#3528)

(Introduced by mistake in flutter/plugins#3509)

Author: ditman

packages/file_selector/file_selector_web/pubspec.yaml

@@ -22,8 +22,8 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
-  mockito: ^5.0.0-nullsafety.5
-  pedantic: ^1.10.0-nullsafety.3
+  mockito: ^4.1.1
+  pedantic: ^1.8.0
   integration_test:
     path: ../../integration_test