Closing fence cannot be followed by tabs

[r-laf]

Hi,

just tested in https://spec.commonmark.org/dingus/ with the following input:

```
abc
```````````	

Notice there is a tab following the closing sequence.

The rendered HTML is :

<pre><code>abc
```````````	
</code></pre>

According to https://spec.commonmark.org/0.30/#fenced-code-blocks, the tab should not affect the closing sequence, so I figure this is a bug?

[jgm]

Yes, thanks for reporting.