Issue for the example #198

Describe the bug
A clear and concise description of what the bug is.
When I use the example that Provided:

from ad_api.api import sponsored_products
my_credentials = dict(
     refresh_token='************************',
      client_id='************************',
      client_secret='************************',
)
result = sponsored_products.Campaigns(credentials=my_credentials).list_campaigns()
print(result)

Get the error:

     self.credentials = CredentialProvider(account, credentials, verify_additional_credentials).credentials
AttributeError: 'CredentialProvider' object has no attribute 'credentials'. Did you mean: 'load_credentials'?

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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