fix property name according to the schema

the schema use index so we have an issue as:

```
Caused by: org.springframework.beans.NotWritablePropertyException: Invalid property 'index' of bean class [org.springframework.integration.splunk.support.SplunkIndexWriter]: Bean property 'index' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?
```