Incorrect comment in get_partition_dispatch_recurse

I noticed that a comment in get_partition_dispatch_recurse claims that:

"it contains the
* leaf partition's position in the global list *leaf_part_oids minus 1"

The "minus 1" part is incorrect. It simply just stores the 0-based
index of the item in the list. I was going to fix it by removing just
that part, but instead, I ended up rewriting the whole thing.

Patch attached.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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