Markdown code block renders language css as c

[elvinn]

Bug report

Version

0.10.0

Steps to reproduce

1. write down below markdown content:

``` css
span {
padding: 20px;
}
```

2. start dev server, then result is as below:
   

What is expected?

show css rather than c in upper right corner.

[ulivz]

Thanks, fixed at 777c4f1