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

(Introduced by mistake in https://github.com/flutter/plugins/pull/3509)
diff --git a/packages/file_selector/file_selector_web/pubspec.yaml b/packages/file_selector/file_selector_web/pubspec.yaml
index 79181e8..a170d5f 100644
--- a/packages/file_selector/file_selector_web/pubspec.yaml
+++ b/packages/file_selector/file_selector_web/pubspec.yaml
@@ -22,8 +22,8 @@
 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