modify the sample value of LOGDAYS

the value of LOGDAYS in samples/local.conf is 2, so change the
value in the comment and the sample value in the document to
be consistent with it.

Change-Id: I5822bbf1d6ad347c67c886be1e3325113d079114
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index dd8f21f..40a8725 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -279,7 +279,7 @@
 
 ::
 
-    LOGDAYS=1
+    LOGDAYS=2
 
 Some coloring is used during the DevStack runs to make it easier to
 see what is going on. This can be disabled with::