[1.6.x] Fixed an instance of 'an' before a word beginning with a consonant.

Backport of ef8ef2a42d from master
[1.6.x] Fixed typo in topics/forms/formsets.txt

Backport of e265112cd1 from master
[1.6.x] Corrected indentation in a contrib.humanize test; refs #23340.

Thanks Zach Borboa for the report.

Backport of 96010ae15b from master