tools: bpftool: add a prog array map update example to documentation

Add an example in map documentation to show how to use bpftool in order
to update the references to programs hold by prog array maps.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>