Update library name

My previous PR erroneously added "Transformers" as library name, but this checkpoint isn't compatible with that.

Instead we can add a dedicated "open-sora" tag and then I'll open a corresponding PR on huggingface.js to enable download metrics for you.

README.md

@@ -1,7 +1,7 @@
 ---
 license: apache-2.0
 pipeline_tag: text-to-video
-library_name: transformers
+library_name: open-sora
 ---
 
 ## Open-Sora: Democratizing Efficient Video Production for All