Remove misplaced backslash from documentation

A \c{} section is already rendered using monospace, no need to escape
the \nullptr keyword explicitly.

Change-Id: I004a409892809e968c7a73c68a5b3c54a4680425
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -392,7 +392,7 @@ void QWidget::setAutoFillBackground(bool enabled)
     Every widget's constructor accepts one or two standard arguments:
 
     \list 1
-        \li  \c{QWidget *parent = \nullptr} is the parent of the new widget.
+        \li  \c{QWidget *parent = nullptr} is the parent of the new widget.
             If it is \nullptr (the default), the new widget will be a window.
             If not, it will be a child of \e parent, and be constrained by
             \e parent's geometry (unless you specify Qt::Window as window flag).