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:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• 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

[cpoerschke]

Thanks for spotting and sending PR to fix! Maybe one more here?

Suggested change

	The `\+` symbol (also known as the "required" operator) requires that the term after the `+` symbol exist somewhere in a field in at least one document in order for the query to return a match.
	The `\+` symbol (also known as the "required" operator) requires that the term after the `\+` symbol exist somewhere in a field in at least one document in order for the query to return a match.

[malliaridis]

I thought so too, but apparently because the sentence ends without another quoted + sign, ASCIIDoc recognizes the second one correctly. Escaping it would also display the escape character together with the + sign. Weird behavior if you ask me.