stall-analyser: print_graph: fix off by one error regarding ommitted branches
stall-analyser: print_graph: fix off by one error regarding ommitted branches

`omitted_idx` and `last_idx` are 1-based, not 0-based.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar