Help: Fix wrong property name INTERFACE_PROPERTY_LINK_DIRECT

This is the name of a placeholder used for some documentation in
`Help/prop_tgt/INTERFACE_LINK_LIBRARIES_DIRECT.txt` that is included in
multiple documents.  During development iterations, much of the content
was moved to just `Help/prop_tgt/INTERFACE_LINK_LIBRARIES_DIRECT.rst`,
but the placeholder was not correctly replaced by its property name.