Fixes#11015
On Windows the location of the example files is "c:\program files\grampsaio64-5.0.1\share\doc\gramps\example\gramps".
On my Ubuntu install from the .deb file these files are in "/usr/share/doc/gramps/example/gramps"
Since the default GRAMPS_RESOURCES value for each system is the 'share', to find the media files we need to append "/doc/gramps/example/gramps" to have the example files work properly.