[4.0] Show help for system menuitems

[Bakual]

When you try to use the "Help" button for a system menuitem like "Menu Heading" or "URL" you get a generic help screen instead of the one for the menuitemtype

Summary of Changes

The lookup for the XML was looking in the wrong path for system menuitems.

Testing Instructions

Try the help button for "System Links" menuitem types

Actual result BEFORE applying this Pull Request

Generic help

Expected result AFTER applying this Pull Request

Specific help

Documentation Changes Required

[brianteeman]

I have tested this item ✅ successfully on b0918e3

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/35523.}

[ChristineWk]

I have tested this item ✅ successfully on b0918e3

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/35523.}

[chmst]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/35523.}

[wilsonge]

Thanks!