Fix case of SpecialShortPages in SpecialPageFactory

[autoloader] Class ShortpagesPage was loaded using incorrect case

Just for information: This was not visible on Special:Specialpages.
In SpecialPageFactory::$list the class LongPagesPage is before
ShortPagesPage and loads that class due to "extends", when
ShortPagesPage should be loaded, the class is already there and no
warning is generated. The warning is shown, when reording the $list.

Change-Id: Ie0ac708b40ff51372545278bb694865650fa8b1f