Example block has no format in Asciidoc

Summary

Example blocks in Asciidoc doesn't render in any different way than normal paragraphs.

Steps to reproduce

1. Create a simple asciidoc readme file like this:

.Example block

This is an example block.

```

2. Push to gitlab and see the result. The block is rendered like a normal paragraph.

You can check with asciidoctor or any other asciidoc renderer that example blocks have a particular style.

Alternatively, you can check this by looking at this example project: https://gitlab.com/joanq/asciidocexampleblockbug

What is the current bug behavior?

The block is rendered like a normal paragraph.

What is the expected correct behavior?

There should be some kind of style, similar to an admonition block without icon.

Output of checks

This bug happens on GitLab.com