SOLR-17358: Escape + symbol to render documentation correctly

[malliaridis]

https://issues.apache.org/jira/browse/SOLR-17358

Description

Fix minor rendering issue to show + symbol again in documentation.

Note that escaping with double backticks doesn't seem to work in this scenario.

Checklist

Please review the following and check all that apply:

• [x] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [x] I have created a Jira issue and added the issue ID to my pull request title.
• [x] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• [x] I have developed this patch against the main branch.
• [ ] I have run ./gradlew check.
• [ ] I have added tests for my changes.
• [ ] I have added documentation for the Reference Guide