Update `base_model_relation` to `finetune`

Hello!

## Pull Request overview
* Update `base_model_relation` to `finetune`

## Details
The Hugging Face Hub sees this model as a `quantized` model instead of a `finetune`, because it has `onnx` files, so it says:


Instead of listing this as a finetune. This PR should fix that :)

- Tom Aarsen

README.md

@@ -4,6 +4,7 @@ language:
 - en
 base_model:
 - answerdotai/ModernBERT-base
+base_model_relation: finetune
 pipeline_tag: sentence-similarity
 library_name: transformers
 tags: