Fix grammar: pluralize user accounts

[ritik35k]

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Source	Review	Production	What Changed
get-started/learning-about-github/types-of-github-accounts.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

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[Sharra-writes]

@ritik35k I think the way it is might be technically correct, because the "are" refers to "types" rather than "account," but I also don't think it matters enough to go on a grammatical hunt for, so I'll just accept this because I think it reads more comfortably.

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