Fix CHANGELOG format [ci skip]

* Add backticks
* Expand tabs
* Fix indentation

activejob/CHANGELOG.md

-*   Allow block to be passed to ActiveJob::Base.discard_on to allow custom handling of discard jobs.
+*   Allow block to be passed to `ActiveJob::Base.discard_on` to allow custom handling of discard jobs.
 
     Example:
 
         class RemoteServiceJob < ActiveJob::Base
-      	   discard_on(CustomAppException) do |job, exception|
-      	     ExceptionNotifier.caught(exception)
-           end
+          discard_on(CustomAppException) do |job, exception|
+            ExceptionNotifier.caught(exception)
+          end
 
           def perform(*args)
             # Might raise CustomAppException for something domain specific
@@ -14,6 +14,7 @@
 
     *Aidan Haran*
 
+
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
 *   No changes.