Use SetInitialSize() in the about wxHTML sample.

SetSize() is useless here as it is simply ignored by the sizer which will fit
the window to its initial size anyhow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76430 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775