Fix some typos with {a,an}

Michael Paquier <michael@paquier.xyz>

Commit: a04fc56d03b33780612cf7630bb3f1a9388a9a02
Author: Michael Paquier <michael@paquier.xyz>
Date: 2021-12-09T06:20:45Z
Releases: 14.2
Fix some typos with {a,an}

One of the changes impacts the documentation, so backpatch.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Pu6+c+r3mY24VT7u+H+E_s6vMr5OdRiZ8NT3EOa-E5Lmw@mail.gmail.com
Backpatch-through: 14

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