Update references generation docs to exclude index

In #23179 the migration generator was changed to no longer output `index: true` for `references` migrations. This updates the migrations guide to remove `index: true` from relevant examples.

[ci skip]

1 files changed, 2 insertions, 2 deletions

diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md
index f914122242..a45becf670 100644
--- a/guides/source/active_record_migrations.md
+++ b/guides/source/active_record_migrations.md
@@ -241,7 +241,7 @@ generates
 ```ruby
 class AddUserRefToProducts < ActiveRecord::Migration[5.0]
   def change
-    add_reference :products, :user, index: true, foreign_key: true
+    add_reference :products, :user, foreign_key: true
   end
 end
 ```
@@ -313,7 +313,7 @@ will produce a migration that looks like this
 class AddDetailsToProducts < ActiveRecord::Migration[5.0]
   def change
     add_column :products, :price, :decimal, precision: 5, scale: 2
-    add_reference :products, :supplier, polymorphic: true, index: true
+    add_reference :products, :supplier, polymorphic: true
   end
 end
 ```