Fix pipeline tag

This PR changes the pipeline tag to text-to-video, to reflect the functionality more closely.

README.md

@@ -1,7 +1,7 @@
 ---
-license: mit
 library_name: diffusers
-pipeline_tag: image-to-video
+license: mit
+pipeline_tag: text-to-video
 ---
 
 <p align="center">
@@ -99,4 +99,4 @@ The following table shows the requirements for running Step-Video-TI2V model (ba
       primaryClass={cs.CV},
       url={https://arxiv.org/abs/2502.10248}, 
 }
-```
+```