Doc: rename the end file for serialport docs

This page has been showing up in the master Qt docs as
http://qt-project.org/doc/qt-5/examples.html, which is way too
generic. The current practice is to add the module:

    \page qt3d-examples.html
    \page qt3d-examples-placeholder.html
    \page examples-graphicsview.html
    \page examples-graphicsview.html
    \page bluetooth-examples.html
\page nfc-examples.html
\page qtquick-codesamples.html
    \page qtexamplesandtutorials.html
    \page examples-layouts.html
    \page examples-draganddrop.html
    \page examples-threadandconcurrent.html
    \page examples-sql.html
    \page examples-xml.html
    \page examples-statemachine.html
    \page examples-animation.html
    \page examples-gestures.html
    \page examples-activeqt.html
  \page qdeclarativeexamples.html
    \page declarative-cppextensions-reference.html
    \page declarative-cppextensions-qgraphicslayouts.html
    \page examples-serialport.html
    \page qtwebkitexamples-index.html

Change-Id: Ib272f6380cb694ad4acfc24a248266805cef05a8
Reviewed-by: Martin Smith <martin.smith@digia.com>