Update pipeline tag

This PR enables people to discover the model easier when https://github.com/huggingface/huggingface.js/pull/798 gets merged.

README.md

@@ -1,3 +1,4 @@
----
-license: apache-2.0
----
+---
+license: apache-2.0
+pipeline_tag: video-text-to-text
+---