Fix for line of credit

[WoodWireAndFood]

The line of credit account type is mis-capitalized in the AccountType class. Here's a fix:

diff --git a/ynab/models.py b/ynab/models.py
index 6013a3e..9ab79b0 100644
--- a/ynab/models.py
+++ b/ynab/models.py
@@ -17,7 +17,7 @@ class AccountType(Enum):
     SAVINGS = 'Savings'
     CREDIT_CARD = 'CreditCard'
     CASH = 'Cash'
-    LINE_OF_CREDIT = 'LineOfCredit'
+    LINE_OF_CREDIT = 'LineofCredit'
     PAYPAL = 'Paypal'
     MERCHANT_ACCOUNT = 'MerchantAccount'
     INVESTMENT_ACCOUNT = 'InvestmentAccount'