update codeowners, to be more specific

[jxs]

Issue Addressed

I keep being notified for PR's like #7009 where it doesn't touch the specified directories in the CODEOWNERS file.
After reading the docs not having a forward slash in beginning of the path means:

In this example, @octocat owns any file in an apps directory
anywhere in your repository.

whereas with the slashes:

In this example, @doctocat owns any file in the /docs
directory in the root of your repository and any of its
subdirectories.

this update makes it more rigid for the files in the jxs ownership

[michaelsproul]

I think the reason you were notified for 7009 is that we changed the target branch, which brought in everything from unstable to the PR, and then we rebased so the commits from unstable were no longer part of the PR

It might be better if we rebase first and then change the target branch, but github actions has a quirk where it won't re-trigger the target branch check when the target changes, meaning another rebase/dummy commit is required to get it to update