Correct pipeline tag

This PR ensures the model can be viewed at https://huggingface.co/models?pipeline_tag=text-to-image&sort=trending

README.md

@@ -1,7 +1,7 @@
 ---
+pipeline_tag: text-to-image
 license: apache-2.0
 tags:
-- Text-to-Image
 - Non-Autoregressive
 ---