[Fix / Boleto]: Fix missing split parameter for creating split "Boleto"

[GuilhermeCaruso]

What does this PR do?

Fix missing parameters in Boleto entity.

It's worth remembering that this information appears incorrectly in the documentation HERE

What motivated the PR?

When trying to execute the create method from Boleto module, the error below occurred

TypeError: Boleto.__init__() got an unexpected keyword argument 'splits'

How to use implemented changes

boletos = starkbank.boleto.create([
    starkbank.Boleto(
        amount=400000,
        due="2020-05-20",
        name="Iron Bank S.A.",
        tax_id="20.018.183/0001-80",
        fine=2.5,
        interest=1.3,
        overdue_limit=5,
        street_line_1="Av. Faria Lima, 1844",
        street_line_2="CJ 13",
        district="Itaim Bibi",
        city="São Paulo",
        state_code="SP",
        zip_code="01500-000",
        tags=["War supply", "Invoice #1234"],
        discounts=[
            {"percentage": 10, "date": "2020-04-25"}
        ],
        
       '''
       Now Boleto class has the splits parameter
       '''
        
        splits=[
            Split(amount=3000, receiverId="5742447426535424"),
            Split(amount=5000, receiverId="5743243941642240")
        ]
    )
])