Fix a typo

Author: sunmeng

src/gui/util/qundostack.cpp

@@ -830,7 +830,7 @@ int QUndoStack::index() const
 /*!
     Repeatedly calls undo() or redo() until the current command index reaches
     \a idx. This function can be used to roll the state of the document forwards
-    of backwards. indexChanged() is emitted only once.
+    or backwards. indexChanged() is emitted only once.
 
     \sa index(), count(), undo(), redo()
 */