Instantiate org-drawer instead of org-basic-drawer.

[aartaka]

This replaces the non-existent org-basic-drawer class name with the existing org-drawer.

Backstory: I tried to parse a file with drawers and a debugger shouted this at me:

There is no class named CL-ORG-MODE::ORG-BASIC-DRAWER.
   [Condition of type SB-PCL:CLASS-NOT-FOUND-ERROR]

org-basic-drawer is not mentioned anywhere but in the patched piece of code and in doc/raw-parser.org, so no hurt in replacing it with an existing class. @deepfire, was it a plan for future to subclass org-basic-drawer from org-drawer?