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/samples/local.conf b/samples/local.conf
index 8b76137..55b7298 100644
--- a/samples/local.conf
+++ b/samples/local.conf
@@ -49,7 +49,7 @@
 # path of the destination log file.  A timestamp will be appended to the given name.
 LOGFILE=$DEST/logs/stack.sh.log
 
-# Old log files are automatically removed after 7 days to keep things neat.  Change
+# Old log files are automatically removed after 2 days to keep things neat.  Change
 # the number of days by setting ``LOGDAYS``.
 LOGDAYS=2