Doc: Fix variable name in logging config example

$LOGDIR --> $DEST

Change-Id: I461422f48525edf4ecb388b2f962edb03795db50

Author: ArtofBugs

doc/source/configuration.rst

@@ -323,7 +323,7 @@ a file, keep service logs and disable color in the stored files.
 
    [[local|localrc]]
    DEST=/opt/stack/
-   LOGFILE=$LOGDIR/stack.sh.log
+   LOGFILE=$DEST/stack.sh.log
    LOG_COLOR=False
 
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