Fix typo

@j-mutter @karlhungus

Nokogiri doesn't have an `ident` option.

indent output:
```xml
<?xml version="1.0"?>
<createTransactionRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<merchantAuthentication>
<name>name</name>
<transactionKey>key</transactionKey>
</merchantAuthentication>
</createTransactionRequest>
```

ident output:
```xml
<?xml version="1.0"?>
<createTransactionRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
  <merchantAuthentication>
    <name>name</name>
    <transactionKey>key</transactionKey>
  </merchantAuthentication>
</createTransactionRequest>
```