fix: incorrect code snippet regex match

[yuyinws]

resolve #2087

Description

When the code snippet syntax does not specify a specific language, the content below will be incorrectly matched, for example:

<<< @/file-a.js

```bash
# Testing
```

Before match result:

After this PR:

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commit: 4c8d36e

[kraenhansen]

Yay 🙌