Fix documentation for pg_stat_recovery_prefetch.

Fix documentation for pg_stat_recovery_prefetch.

The table was in the wrong section and using an older type of link, as
reported by Sirisha, and also using an older table layout, as I noticed
while trying to figure out how and when it might have got out of sync.

Defect in commit 5dc0418f.

Author: sirisha chamarthi <sirichamarthi22@gmail.com>
Author: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CAKrAKeVk-LRHMdyT6x_p33eF6dCorM2jed5h_eHdRdv0reSYTA@mail.gmail.com