Fix mention of htup.h in storage.sgml

Fix mention of htup.h in storage.sgml

Previously it was documented that the details on HeapTupleHeaderData
struct could be found in htup.h. This is not correct because it's now
defined in htup_details.h.

Back-patch to 9.3 where the definition of HeapTupleHeaderData struct
was moved from htup.h to htup_details.h.

Michael Paquier