readme fix: Shipment.all – pass parameters as a hash

[Jonathan-Jacob]

Replaced this

Shipcloud::Shipment.all(
  carrier: 'ups',
  per_page: 25,
  page: 2
)

with this

Shipcloud::Shipment.all({
  carrier: 'ups',
  per_page: 25,
  page: 2
})

since otherwise you will get an ArgumentError

[Jonathan-Jacob]

Failed checks should be irrelevant since this is only a PR for a README change