Correct the name of Doxyfile containing the version.

Use the correct file name in the version update script and version update
instructions, it is called just Doxyfile and not Doxyfile_inc now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75260 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775