ScientificLinux : Use of correct grain value in modules/service.py

Small bug fix.
modules/service.py should use correct grain value for ScientificLinux
as reported below.

# salt-call grains.item os
local:
  os: ScientificLinux
Merge pull request #12059 from mlister2006/develop

ScientificLinux : Use of correct grain value in modules/service.py