doc: fix XML for the RNG device example
doc: fix XML for the RNG device example

Add a missing '/' to close the "source" element.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

fabianfreyer/libvirt,shugaoye/libvirt,cbosdo/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,datto/libvirt,zippy2/libvirt,nertpinx/libvirt,agx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,agx/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,datto/libvirt,nertpinx/libvirt,libvirt/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,olafhering/libvirt,olafhering/libvirt,taget/libvirt,andreabolognani/libvirt,eskultety/libvirt,VenkatDatta/libvirt,olafhering/libvirt,fabianfreyer/libvirt,agx/libvirt,eskultety/libvirt,elmarco/libvirt,nertpinx/libvirt,elmarco/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,crobinso/libvirt,cbosdo/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,libvirt/libvirt,agx/libvirt,shugaoye/libvirt,crobinso/libvirt,eskultety/libvirt,rlaager/libvirt,shugaoye/libvirt,rlaager/libvirt,fabianfreyer/libvirt,elmarco/libvirt,taget/libvirt,jardasgit/libvirt,rlaager/libvirt,libvirt/libvirt,shugaoye/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,eskultety/libvirt,cbosdo/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,rlaager/libvirt,andreabolognani/libvirt,zippy2/libvirt,taget/libvirt,jardasgit/libvirt,jardasgit/libvirt,elmarco/libvirt,eskultety/libvirt,datto/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,datto/libvirt,crobinso/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,agx/libvirt,cbosdo/libvirt,taget/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,taget/libvirt