Revert "[ci skip] Fix to `a, b or c` format"

This reverts commit 9876a169.

Reason: Oxford commas are preferred. This is now documented in
the documentation guidelines.

guides/source/active_support_core_extensions.md

@@ -3303,7 +3303,7 @@ Date.new(2010, 2, 28).advance(days: 1).advance(months: 1)
 
 #### Changing Components
 
-The method `change` allows you to get a new date which is the same as the receiver except for the given year, month or day:
+The method `change` allows you to get a new date which is the same as the receiver except for the given year, month, or day:
 
 ```ruby
 Date.new(2010, 12, 23).change(year: 2011, month: 11)