[IOT-1353] Fixed README file for account server build

[IOT-1353] Fixed README file for account server build

README file fixed to resolve cloud account server build issue

Change-Id: Iaa4e61480a54dd9c5b7d66d5a5c23e77e5fb8d77
Signed-off-by: Jongmin Choi <jminl.choi@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12401
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jee Hyeok Kim <jihyeok13.kim@samsung.com>
(cherry picked from commit 567ff432a0d79e670b82e75eb7e0238bbd2455c4)
Origin: https://gerrit.iotivity.org/gerrit/#/c/15271/
Reviewed-on: https://gerrit.iotivity.org/gerrit/15271
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>