Fix an incorrect word in a pubspec validation error message.

sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart

Index: sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart b/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
index 7ff8ec77c459ae1c04fba2ff45c1d03d44f62267..546eb26c7e5b26954b109ab77a890984756f3eaa 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
@@ -54,7 +54,7 @@ class PubspecFieldValidator extends Validator {
 
     if (authors != null &&
         (authors is! List || authors.any((author) => author is! String))) {
-      errors.add('Your pubspec.yaml\'s "authors" field must be a string, but '
+      errors.add('Your pubspec.yaml\'s "authors" field must be a list, but '
           'it was "$authors".');
       return;
     }