Remove Rdoc formatting from markdown [ci skip]

Use markdown formatting instead

guides/source/active_support_core_extensions.md

@@ -424,7 +424,7 @@ NOTE: Defined in `active_support/core_ext/object/with_options.rb`.
 
 ### JSON support
 
-Active Support provides a better implementation of `to_json` than the +json+ gem ordinarily provides for Ruby objects. This is because some classes, like +Hash+, +OrderedHash+, and +Process::Status+ need special handling in order to provide a proper JSON representation.
+Active Support provides a better implementation of `to_json` than the `json` gem ordinarily provides for Ruby objects. This is because some classes, like `Hash`, `OrderedHash` and `Process::Status` need special handling in order to provide a proper JSON representation.
 
 NOTE: Defined in `active_support/core_ext/object/json.rb`.