Update help for renamed documentation index

Changelog: other

app/controllers/help_controller.rb

@@ -13,7 +13,7 @@ class HelpController < ApplicationController
 
   def index
     # Remove YAML frontmatter so that it doesn't look weird
-    @help_index = File.read(Rails.root.join('doc', 'README.md')).sub(YAML_FRONT_MATTER_REGEXP, '')
+    @help_index = File.read(Rails.root.join('doc', 'index.md')).sub(YAML_FRONT_MATTER_REGEXP, '')
 
     # Prefix Markdown links with `help/` unless they are external links.
     # '//' not necessarily part of URL, e.g., mailto:mail@example.com

spec/controllers/help_controller_spec.rb

@@ -274,7 +274,7 @@ RSpec.describe HelpController do
   end
 
   def stub_readme(content)
-    expect_file_read(Rails.root.join('doc', 'README.md'), content: content)
+    expect_file_read(Rails.root.join('doc', 'index.md'), content: content)
   end
 
   def stub_two_factor_required