Lowercase birefnet in model card metadata

To make it work with https://github.com/huggingface/huggingface.js/pull/799. The library name will be correctly displayed as `BiRefNet` once detected as a library (requires to merge the PR + get it deployed).

README.md

@@ -1,5 +1,5 @@
 ---
-library_name: BiRefNet
+library_name: birefnet
 tags:
 - background-removal
 - mask-generation