Fix a typo in the Object docs (exception -> except)

Change-Id: Ie160e4b7daa2f989e23edf9f5248b612681d285e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204262
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
diff --git a/sdk/lib/core/object.dart b/sdk/lib/core/object.dart
index 429eba0..32b52bf 100644
--- a/sdk/lib/core/object.dart
+++ b/sdk/lib/core/object.dart
@@ -4,7 +4,7 @@
 
 part of dart.core;
 
-/// The base class for all Dart objects exception `null`.
+/// The base class for all Dart objects except `null`.
 ///
 /// Because `Object` is a root of the non-nullable Dart class hierarchy,
 /// every other non-`Null` Dart class is a subclass of `Object`.