Fix README typo on Related usage.

README.md

@@ -570,7 +570,7 @@ db.Model(&user).Related(&card, "CreditCard")
 //// SELECT * FROM credit_cards WHERE user_id = 123; // 123 is user's primary key
 // CreditCard is user's field name, it means get user's CreditCard relations and fill it into variable card
 // If the field name is same as the variable's type name, like above example, it could be omitted, like:
-db.Model(&user).Related(&creditCard, "CreditCard")
+db.Model(&user).Related(&card)
 ```
 
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