ovsdb-doc: Convert '-' preceding a number as a minus sign, not a hyphen.

ovsdb-doc: Convert '-' preceding a number as a minus sign, not a hyphen.

ovs-vswitchd.conf.db.5 contains the following sentence:

   If the interface cannot be added then Open vSwitch sets this column
   to -1.

The '-' in "-1" should be a minus sign, not a hyphen, but the heuristic
in ovsdb-doc wasn't smart enough.  This commit improves the heuristic and
fixes the problem.

Found by lintian.

Reported-by: Thomas Goirand <zigo@debian.org>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>