Accept 'platforms' property in pubspec.yaml (#3176)

diff --git a/lib/src/validator/pubspec_typo.dart b/lib/src/validator/pubspec_typo.dart
index d0b82b8..a770b76 100644
--- a/lib/src/validator/pubspec_typo.dart
+++ b/lib/src/validator/pubspec_typo.dart
@@ -74,4 +74,5 @@
   'false_secrets',
   'flutter',
   'screenshots',
+  'platforms',
 ];