XBEL streaming: replace images with up-to-date version

The old screenshot showed an old XBEL file's content, with trolltech
URLs. Update to match the new XBEL file's content.

Task-number: QTBUG-111228
Change-Id: Idacc31b7786b1e6ed1425857470b1d37227096cd
Reviewed-by: Kimmo Leppälä <kimmo.leppala@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit a028e53fa49c995a7dc06ffaac2be1ebe0923f9c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/serialization/streambookmarks/doc/src/qxmlstreambookmarks.qdoc

@@ -13,7 +13,7 @@
     Exchange Language (XBEL) files using Qt's QXmlStreamReader class
     for reading, and QXmlStreamWriter class for writing the files.
 
-    \image xmlstreamexample-screenshot.png
+    \image screenshot.png
 
     \section1 XbelWriter Class Definition
 
@@ -153,8 +153,8 @@
 
     \table
     \row
-         \li \inlineimage xmlstreamexample-filemenu.png
-         \li \inlineimage xmlstreamexample-helpmenu.png
+         \li \inlineimage filemenu.png
+         \li \inlineimage helpmenu.png
     \endtable
 
     \snippet serialization/streambookmarks/mainwindow.cpp 5