Update SidebarMenu.ss

Use <% if LinkOrSection = section %> to limit the list of sub-pages to just children of the current page
Reference: http://doc.silverstripe.org/framework/en/2.4/reference/built-in-page-controls 
Thank you for creating this theme.
Merge pull request #1 from AntonyThorpe/patch-1

Update SidebarMenu.ss with <% if LinkOrSection = section %> to limit the list of sub-pages - from AntonyThorpe