Update CHANGELOG

CHANGELOG.md

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 ### Enhancements
 * Support `Realm.logger.level` change at runtime. Added new class `RealmLogger` for managing the realm log ([#1226](https://github.com/realm/realm-dart/pull/1226)).
+* Add `RealmResults.isValid` ([#1231](https://github.com/realm/realm-dart/pull/1231)).
 * Add logging at the Storage level (Core upgrade).
 * Performance improvement for the following queries (Core upgrade):
- * Significant (~75%) improvement when counting (query count) the number of exact matches (with no other query conditions) on a String/int/Uuid/ObjectId property that has an index. This improvement will be especially noticiable if there are a large number of results returned (duplicate values).
+ * Significant (~75%) improvement when counting (query count) the number of exact matches (with no other query conditions) on a String/int/Uuid/ObjectId property that has an index. This improvement will be especially noticeable if there are a large number of results returned (duplicate values).
  * Significant (~99%) improvement when querying for an exact match on a Timestamp property that has an index.
  * Significant (~99%) improvement when querying for a case insensitive match on a Mixed property that has an index.
  * Moderate (~25%) improvement when querying for an exact match on a Boolean property that has an index.