Fix config file documentation

The documentation currently uses '[default]' (lower case).
This configuration is not picket up.
It needs to be upper case.

Change-Id: Icbc5e7ee5178be1054b045045245d384a3957a63
Signed-off-by: Andreas Florath <Andreas.Florath@telekom.de>

README.rst

@@ -33,7 +33,7 @@ You must define what sources to collect configuration data from in
 
 The format of this file is::
 
-  [default]
+  [DEFAULT]
   command=os-refresh-config
 
   [cfn]