Fix image reference in tutorial 4
Fix image reference in tutorial 4

Signed-off-by: Laurent Stacul <laurent.stacul@gmail.com>

Closes: #2059
Approved by: rhatdan

projectatomic/buildah,nalind/buildah-1,nalind/buildah,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah,rhatdan/buildah,nalind/buildah-1