Fix typo in SafeValidator

assigment -> assignment

framework/validators/SafeValidator.php

@@ -14,7 +14,7 @@ namespace yii\validators;
  * when a user submits form data to be loaded into a model directly from the POST data, is it ok for a property to be copied.
  * In many cases, this is required but because sometimes properties are internal and you do not want the POST data to be able to
  * override these internal values (especially things like database row ids), Yii assumes all values are unsafe for massive assignment
- * unless a validation rule exists for the property, which in most cases it will. Sometimes, however, an item is safe for massive assigment but
+ * unless a validation rule exists for the property, which in most cases it will. Sometimes, however, an item is safe for massive assignment but
  * does not have a validation rule associated with it - for instance, due to no validation being performed, in which case, you use this class
  * as a validation rule for that property. Although it has no functionality, it allows Yii to determine that the property is safe to copy.
  *