Docs - Ubuntu - Fix ambiguity

Change username.

Someone testing put username in .xsession instead of sugar.

Author: quozl

docs/ubuntu.md

@@ -35,8 +35,8 @@ Install the Remote Desktop packages:
 
 Create a user for Sugar and set a default desktop environment:
 
-    sudo adduser sugar
-    sudo su - sugar -c 'echo sugar >> .xsession'
+    sudo adduser guest
+    sudo su - guest -c 'echo sugar >> .xsession'
 
 Start a session: