Update BLEURT hyperlink
The description of BLEURT on the hf.co website has a strange use of URL hyperlinking. This PR attempts to fix this, although I am not 100% sure Markdown syntax is allowed on the frontend or not.

![Screen Shot 2021-12-15 at 17 31 27](https://user-images.githubusercontent.com/26859204/146226432-c83cbdaf-f57d-4999-b53c-85da718ff7fb.png)


