Markdown: double-counting of a comment

[mykter]

Given a markdown file:

```r    ```

tokei reports:

$ tokei test.md
===============================================================================
 Language            Files        Lines         Code     Comments       Blanks
===============================================================================
 Markdown                1            2            0            2            0
 |- R                    1            0            0            0            0
 (Total)                              2            0            2            0
===============================================================================
 Total                   1            2            0            2            0
===============================================================================

I found this through fuzzing, I'll raise a PR with the fuzz harness shortly.