Fix typo in build description for Ubuntu

Change-Id: Ida41dae886074e8a728438da2333c29690f4cdca
Reviewed-on: http://review.couchbase.org/40288
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>