Fix documentation of `index` option for the `add_reference` method [ci skip]

- Followup of https://github.com/rails/rails/pull/23179.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -790,7 +790,7 @@ def index_name_exists?(table_name, index_name, default)
       # [<tt>:type</tt>]
       #   The reference column type. Defaults to +:integer+.
       # [<tt>:index</tt>]
-      #   Add an appropriate index. Defaults to false.
+      #   Add an appropriate index. Defaults to true.
       #   See #add_index for usage of this option.
       # [<tt>:foreign_key</tt>]
       #   Add an appropriate foreign key constraint. Defaults to false.