Consider new example for purchase order

[jpmckinney]

https://standard.open-contracting.org/staging/1.2-dev/en/guidance/map/awards_contracts/#purchase-orders

As discussed in #896 (comment), I'm not sure that this scenario would actually involve purchase orders (though purchase orders are a flexible term):

The UK's Department for Transport awards a £1.2m, 12-month contract to KPMG to provide the Project Management Office function for a project to construct a new highway bypass. The contract specifies that payment will be made quarterly in arrears in four equal amounts.

We left the example when making changes in #1338, because the point of that section is to illustrate how double-counting can occur where purchase orders are involved – it is not to show how to model purchase orders.

That said, it would be less confusing to use a more typical example.