TTL KT generates warnings on website build

This happens on the commit introduced by https://github.com/confluentinc/kafka-tutorials/pull/761

```
[12:26:25] ~/git/kafka-tutorials(master) ✗: (cd ~/git/kafka-tutorials && bundle exec jekyll serve --livereload --trace)
Configuration file: /Users/yeva/git/kafka-tutorials/_config.yml
Source: /Users/yeva/git/kafka-tutorials
Destination: /Users/yeva/git/kafka-tutorials/_site
Incremental build: disabled. Enable with --incremental
Generating...
asciidoctor: WARNING: : line 40: no callout found for <1>
asciidoctor: WARNING: : line 41: no callout found for <2>
asciidoctor: WARNING: : line 43: no callout found for <4>
asciidoctor: WARNING: : line 60: no callout found for <3>
done in 4.705 seconds.
Auto-regeneration: enabled for '/Users/yeva/git/kafka-tutorials'
LiveReload address: http://127.0.0.1:35729
Server address: http://127.0.0.1:4000
Server running... press ctrl-c to stop.
```

Note:

```
asciidoctor: WARNING: : line 40: no callout found for <1>
asciidoctor: WARNING: : line 41: no callout found for <2>
asciidoctor: WARNING: : line 43: no callout found for <4>
asciidoctor: WARNING: : line 60: no callout found for <3>
```

cc: @sarwarbhuiyan in case you know what the issue may be