Accept 'screenshots' property in pubspec.yaml (#3174)

diff --git a/lib/src/validator/pubspec_typo.dart b/lib/src/validator/pubspec_typo.dart
index 2e10ed6..d0b82b8 100644
--- a/lib/src/validator/pubspec_typo.dart
+++ b/lib/src/validator/pubspec_typo.dart
@@ -73,4 +73,5 @@
   'publish_to',
   'false_secrets',
   'flutter',
+  'screenshots',
 ];