Doc: corrected qdoc list syntax

qundostack.cpp: Command '\li' outside of '\list' and '\table'

Change-Id: Ic162e246c754e125f6ae82a2a66312e925b231c2
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/widgets/util/qundostack.cpp

@@ -654,10 +654,12 @@ void QUndoStack::setClean()
     This method resets the clean index to -1.
 
     This is typically called in the following cases, when a document has been:
+    \list
     \li created basing on some template and has not been saved,
         so no filename has been associated with the document yet.
     \li restored from a backup file.
     \li changed outside of the editor and the user did not reload it.
+    \endlist
 
     \sa isClean(), setClean(), cleanIndex()
 */