Fix method name in outdated documentation

This method was renamed in the past (#683), but the relevant documentation
still has the old method name.

Co-authored-by: Cristina <[email protected]>

Author: jkempster34

Diff for: docs/deleting-content.md

@@ -25,7 +25,7 @@ class RemoveYourEdition < ActiveRecord::Migration
   def up
     editions = Edition.where(id: 123)
 
-    Helpers::DeleteContent.destroy_supporting_objects(editions)
+    Helpers::DeleteContent.destroy_edition_supporting_objects(editions)
 
     editions.destroy_all
   end