Fix missing article in Active Support docs [ci skip]

guides/source/active_support_core_extensions.md

@@ -180,7 +180,7 @@ array     # => ['foo']
 duplicate # => ['foo', 'another-string']
 ```
 
-As you can see, after duplicating the `Array` instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since `dup` does not make deep copy, the string inside the array is still the same object.
+As you can see, after duplicating the `Array` instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since `dup` does not make a deep copy, the string inside the array is still the same object.
 
 If you need a deep copy of an object, you should use `deep_dup`. Here is an example: