Correct markup in postinstall message

[SharkyKZ]

Pull Request for Issue # .

Summary of Changes

Corrects markup in language string.

Testing Instructions

Code review.

Actual result BEFORE applying this Pull Request

Stray </p> tag.

Expected result AFTER applying this Pull Request

No stray tags.

Documentation Changes Required

No.

[Quy]

I have tested this item ✅ successfully on ca8c03c

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/30663.}

[ChristineWk]

I have tested this item ✅ successfully on ca8c03c

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/30663.}

[Quy]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/30663.}

[HLeithner]

Thanks