Fix issue #48, missing '\' on '\s' perl regexp

With this fix:

$ uptime 
11:55:38 up 189 days, 20 min,  3 users,  load average: 0.06, 0.08, 0.04

$ ./check_uptime.pl -f
OK: Linux myhost.domain.com 2.6.32-358.18.1.el6.x86_64 - up 189 days 20 minutes | type=1 uptime_minutes=272180