Fix URL mapping for CaseStudy now it's migrated to StandardEdition

When we bulk update the organisations that are tagged to a given
CSV of documents, we use this BasePathHelper to help with looking
up the Edition based on the document's URL. It saves the publisher
from having to provide the document types ahead of time.

CaseStudy was recently moved to StandardEdition so its mapping
needs updating accordingly.

Author: ChrisBAshton

app/helpers/admin/base_path_helper.rb

@@ -3,7 +3,7 @@
 module Admin::BasePathHelper
   MAPPINGS = [
     { "CallForEvidence" => "/government/calls-for-evidence" },
-    { "CaseStudy" => "/government/case-studies" },
+    { "StandardEdition" => "/government/case-studies" },
     { "Consultation" => "/government/consultations" },
     { "DetailedGuide" => "/guidance" },
     { "DocumentCollection" => "/government/collections" },