Added mask token to model card widget

More info here: https://github.com/huggingface/transformers/issues/11884

README.md

@@ -4,6 +4,7 @@ language:
 tags:
 - medical
 pipeline_tag: fill-mask
+mask_token: "[MASK]"
 widget:
 - text: "This research study is studying a combination of drugs as a possible treatment for metastatic triple-negative [MASK] cancer."
   example_title: "Trial Summary"