update user name cl_sos

verified again, with user name cl-sos. it doesn't work to fix #663 
but cl_sos is working, so update the user name to cl_sos.  that means 
clear has naming rules for user name

Author: ailun258

doc/getting-started/apl-nuc.rst

@@ -235,16 +235,16 @@ partition. Follow these steps:
 
    .. code-block:: console
 
-      # useradd cl-sos
-      # passwd cl-sos
-      # usermod -G wheel -a cl-sos
+      # useradd cl_sos
+      # passwd cl_sos
+      # usermod -G wheel -a cl_sos
 
 #. Enable weston service
 
    .. code-block:: console
 
-      # systemctl enable weston@cl-sos
-      # systemctl start weston@cl-sos
+      # systemctl enable weston@cl_sos
+      # systemctl start weston@cl_sos
       
 #. Reboot and select "The ACRN Service OS" to boot, as shown below: