[4.0] Fix README.md for unit tests

[richard67]

Pull Request for Issue # .

Summary of Changes

1. Fix typo phpunit-xml instead of phpunit.xml from PR Update README.md for unit tests #32758 .
2. Remove the joomla-cms from paths in the description added by PR Update README.md for unit tests #32758 so that relative paths below the root folder of the checkout are used, like it is already for the given instructions "2. Run composer install in the root of your checkout." and "4. Run libraries/vendor/bin/phpunit".

I should have noticed that when reviewing that PR #32758 , but seems I was sleeping or blind.

Testing Instructions

Review.

Actual result BEFORE applying this Pull Request

Wong file name phpunit-xml instead of phpunit.xml used.

Inconsistent use of paths with and without the checkout root folder joomla-cms.

Expected result AFTER applying this Pull Request

Right file name phpunit.xml used.

Consistent use of relative paths below the checkout root folder`.

Documentation Changes Required

This is the documentation change.

[dgrammatiko]

I have tested this item ✅ successfully on e87f892

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/32935.}

[meinhoonharsh]

I have tested this item ✅ successfully on e87f892

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/32935.}

[alikon]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/32935.}

[astridx]

@richard67 Thanks for correcting my PR.