doc: update tutorial splitting script
doc: update tutorial splitting script

Some recent changes to pandoc's output format broke the script
doc/htmlsplit.py so now "ninja -C build/dev doc/docs" fails.
This patch makes the parsing in htmlsplit.py more lax, to accept
both older and newer output.

Message-Id: <1466009979491779a43b6fa1f6e47717122e21c1@scylladb.com>

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