[Doxygen] Fix dirty repo detection.

doc/ns3_html_theme/get_version.sh

@@ -153,7 +153,7 @@ else
     version=`hg log -r tip --template '{node|short}'`
     # Check for uncommitted changes
     hg summary | grep -q 'commit: (clean)'
-    if [ ! $? ] ; then
+    if [ $? ] ; then
 	say "beyond latest tag, last commit: $version, dirty"
 	dirty="(+)"
     else