[ci skip] Fix typo in deep_dup docs

guides/source/active_support_core_extensions.md

@@ -172,7 +172,7 @@ NOTE: Defined in `active_support/core_ext/object/duplicable.rb`.
 
 ### `deep_dup`
 
-The `deep_dup` method returns deep copy of a given object. Normally, when you `dup` an object that contains other objects, Ruby does not `dup` them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:
+The `deep_dup` method returns a deep copy of a given object. Normally, when you `dup` an object that contains other objects, Ruby does not `dup` them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:
 
 ```ruby
 array     = ['string']