Typos in src/backend/optimizer/README

Hi,

This sentence in the section of Partition-wise joins in 
src/backend/optimizer/README should be fixed: "This technique of 
breaking down a join between partition tables into join between their 
partitions is called partition-wise join."

(1) s/a join between partition tables/a join between partitioned tables/
(2) s/join between their partitions/joins between their partitions/

It might be okay to leave #2 as-is, but I'd like to propose to change 
that way to make the meaning clear.

Attached is a patch for that.

Best regards,
Etsuro Fujita