Update dataset metadata and task categories

This PR updates the YAML metadata of the dataset card to use the standard `task_categories` field (`image-to-text`) instead of `pipeline_tag`, which is typically reserved for models. This will allow the dataset to be properly indexed and discovered on the Hugging Face Hub under the "Image-to-Text" task.

README.md

@@ -1,6 +1,7 @@
 ---
 license: apache-2.0
-pipeline_tag: image-to-text
+task_categories:
+- image-to-text
 ---
 
 # UniRec-0.1B: Unified Text and Formula Recognition with 0.1B Parameters