multi line matches with ripgrep

I had just noticed that I'd accidentally made a code block in one of these here markdown files and forgot to specify the language - like this:

```
some stuff
```

rather than

```language_goes_here
some stuff
```

which for some reason hugo's markdown renderer renders differently. How to find where else I'd made that mistake:

$ rg -U '^\`\`\`\n[^\n]' ./