primops.txt.pp: add missing type application in description of GHC.Prim.Any

Understanding GHC.Prim.Any is all about understanding the need for
explicit type applications in CoreSyn; the previous example had one
explicit type application but was missing the other, which this patch
adds.  It also expands the explanation, based on SPJ's post here:

  http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/46551