Changes

doc: set state-dir to /var/lib/osmocom/osmo-sgsn

Be consistent with osmo-ggsn, and set the state dir to
/var/lib/osmocom/osmo-sgsn. Without this patch, it defaults to the
current directory, which means in case of running with the systemd
service, /var/lib/osmocom.

Related: osmo-ggsn I5b51529b4f8bd2462e54f58a1ce2e2d7c76ff46a
Change-Id: I309807ff0bc125d4653222b2b4ba69ded3bbff70