obsolete comment above index_pages_fetched

Hi,

I think the following commit:

commit c6e4133fae1fde93769197379ffcc2b379845113
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Wed Nov 7 12:12:56 2018 -0500

    Postpone calculating total_table_pages until after pruning/exclusion.
...


obsoleted a sentence in the comment above index_pages_fetched(), which says:

 * "index_pages" is the amount to add to the total table space, which was
 * computed for us by query_planner.

total_table_pages is computed by make_one_rel as of the aforementioned
commit.  Attached fixes this.

Thanks,
Amit