Upate README to match Shopify docs

[matt-cos]

Using value_type as a key in the params object was throwing a 422 error:

{
  "errors": {
    "value_type": [
      "is not valid"
    ]
  }
} 

The Shopify API is expecting type as the key.

Fixes #622

[coveralls]

coverage: 100.0%. remained the same when pulling 0eef8a85920bcc1d1bd8502eef8cb77e3f8c6925 on matt-cos:master into 848b773bad91feac49de5c5336150c4db8c50826 on MONEI:master.

[lpinca]

Thank you.