Fix an incorrect word in a pubspec validation error message.

R=rnystrom@google.com
BUG=17216

Review URL: https://codereview.chromium.org//198063006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@33676 260f80e4-7a28-3924-810f-c04153c831b5

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;
     }