[IO-886] Fixed incorrect regular expression in PathUtils.RelativeSortedPaths.extractKey(String, String)

[peterdemaeyer]

Fixed incorrect regular expression in replaceAll by replacing it with a plain replace.

Thanks for your contribution to Apache Commons! Your help is appreciated!

Before you push a pull request, review this list:

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create any part of, or all of, this pull request. Which AI tool was used to create this pull request, and to what extent did it contribute?
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body. Note that a maintainer may squash commits during the merge process.

[garydgregory]

Hello @peterdemaeyer

Thank you for the PR.

This PR is missing a unit test that fails when the main side of the tree is not applied, which is just asking for a regression IMO.

[garydgregory]

I looked at this again and it makes sense; building...

[garydgregory]

The Jira ticket is https://issues.apache.org/jira/browse/IO-886