Set `library_name` to `tf-keras`.

Model 'keras-io/siamese-contrastive' seems to be compatible only with "Keras 2" and not "Keras 3". To distinguish them, models compatible with legacy Keras 2.x should be tagged as `tf-keras` while models compatible with Keras 3.x are tagged as `keras`.

This PR updates the model card to replace the explicit `library_name: keras` metadata which is now outdated by `library_name: tf-keras`. Updating this metadata will facilitate its discoverability and usage.

For more information about `keras` and `tf-keras` library names, check out this pull request: https://github.com/huggingface/huggingface.js/pull/774.

README.md

@@ -1,5 +1,5 @@
 ---
-library_name: keras
+library_name: tf-keras
 ---
 
 ## Model description