Doc: Minor update for enable_partitionwise_aggregate

Hello. While reading the docs for the enable_partitionwise_aggregate
parameter on the Query Planning page, I thought the description had a small
mistake that could be improved.

The current wording is: "which allows grouping or aggregation on a
partitioned tables performed separately "

Page: https://www.postgresql.org/docs/current/runtime-config-query.html

I think possible better alternatives could be:

- (Option 1) a "partitioned table's partitions" (the possessive form of
"it's"). The "enable_partition_pruning" parameter uses "the partitioned
table's partitions" in this form. I think this option is good, but I had a
slight preference for option 2.
- (Option 2) Or to just cut out the first part and say "to be performed
separately for each partition", which seemed simpler. So the sentence
reads: "which allows grouping or aggregation to be performed separately for
each partition"
- (Option 3) dropping the "a" so it says "which allows grouping or
aggregation on partitioned tables performed separately". I don't think this
is as good though because the aggregation happens on the partitions, so it
feels slightly off to me to say the "partitioned tables" instead of the
partitions.

I tested toggling this parameter on and off with a test partitioned table,
and looked at the query execution plan, and saw how the aggregation
happened on the partitions first when it was enabled.

This is my first ever submission to pgsql-hackers. :) I used this guide
from Lætitia to prepare the patch file for Option 2 above, which is
attached. I am having a problem with the "make STYLE=website html" step, so
I hadn't seen the preview (still fixing this up).
https://mydbanotebook.org/post/patching-doc/

Let me know what you think!

Thanks.