Default value for validation schema has moved

[mikefat]

Currently the default path for the json schema used for validation is: https://github.com/SigmaHQ/sigma-rules-validator/blob/main/action.yaml#L14, but it should be changed as it no longer exists there under main.

It can now can be found here: https://raw.githubusercontent.com/SigmaHQ/sigma-specification/refs/heads/main/json-schema/sigma-detection-rule-schema.json

The schemas have split for files with correlation too - something that will probably help fix issue #3