Update model metadata to set pipeline tag to the new `text-ranking`

Hello!

## Pull Request overview
* Update metadata to set pipeline tag to the new `text-ranking`

## Changes
This is an automated pull request to update the metadata of the model card. We recently introduced the [`text-ranking`](https://huggingface.co/models?pipeline_tag=text-ranking) pipeline tag for models that are used for ranking tasks, and we have a suspicion that this model is one of them.

Feel free to respond if you have questions or concerns.

- Tom Aarsen

README.md

@@ -9,7 +9,7 @@ tags:
 base_model: microsoft/MiniLM-L12-H384-uncased
 datasets:
 - tomaarsen/ms-marco-shuffled
-pipeline_tag: text-classification
+pipeline_tag: text-ranking
 library_name: sentence-transformers
 metrics:
 - map