add missing foreign key index, correct constraint name

[ogobrecht]

Missing foreign key index:

• file: customer_orders/co_tables.sql
• action: add code create index order_items_shipment_id_i on order_items (shipment_id)

Wrong constraint name:

• file: customer_orders/co_constraints.sql
• code (line 65): alter table inventory add constraint shipments_product_id_fk foreign key (product_id) references products (product_id);
• action: replace shipments_product_id_fk with inventory_product_id_fk

[ogobrecht]

You are welcome, thanks for merging my pull request