Fix formula in _hash_spareindex.

This was correct in earlier versions of the patch that lead to
commit ea69a0de, but somehow got
broken in the last version which I actually committed.

Mithun Cy, per an off-list report from Ashutosh Sharma

Discussion: http://postgr.es/m/CAD__OujbAwNU71v1y-RoQxZ8LZ6-V2UFTkex3v34MK6uZ3Xb5w@mail.gmail.com