docs: Fix the icons in documentation of RDF properties

These needed updating after tracker: properties moved to nrl:
in !274/

This also fixes a mismatch of the <entry> tag that was preventing
the icons from appearing.

The icons are still without tooltips as this isn't possible with
gtk-doc[1], making them a bit cryptic. Perhaps we should include
a key somewhere...

[1]: gtk-doc#123
3 jobs for sam/ontology-doc-fixes in 1 minute and 54 seconds
Status Job ID Name Coverage
  Test
passed #788531
test-alpine-edge

00:01:54

passed #788532
test-fedora-latest

00:00:58

passed #788533
test-ubuntu-rolling

00:01:14