doc: Fix some grammar and typos

Michael Paquier <michael@paquier.xyz>

Commit: 83e42a0035718914efad2fe04080fd6c579e9ef2
Author: Michael Paquier <michael@paquier.xyz>
Date: 2022-10-01T06:28:02Z
Releases: 16.0
doc: Fix some grammar and typos

This fixes some areas related to logical replication and custom RMGRs.

Author: Ekaterina Kiryanova
Discussion: https://postgr.es/m/fa4773f1-1396-384a-bcd7-85b5e013f399@postgrespro.ru
Backpatch-through: 15

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