Add video-text-to-text task category

This PR adds the `video-text-to-text` task category to the dataset card so the dataset can also be found at https://huggingface.co/datasets?task_categories=task_categories:video-text-to-text.

README.md

@@ -1,9 +1,11 @@
 ---
-license: cc-by-nc-4.0
 language:
 - zh
+license: cc-by-nc-4.0
 tags:
 - sign-language
+task_categories:
+- video-text-to-text
 ---
 
 # Summary