Update README.md

Tags should be array according to the following error on the screen.
> YAML Metadata Error: "tags" must be an array

So I suggest to fix this issue.

README.md

@@ -1,5 +1,5 @@
 ---
-tags: autonlp
+tags: [autonlp]
 language: ja
 widget:
 - text: "I love AutoNLP 🤗"