Fix: Rename "Post format" category to "Post formats"

[juanfra]

Description

Fixes #658

Renaming the category to "Post formats" (plural), both to fix the typo (there's more than one pattern in the category), and to make it consistent with the other categories.

This involves changing a translatable string, so we should probably involve #polyglots

Screenshots

Testing Instructions

1. Create a page.
2. Open the inserter.
3. Confirm that the category is "Post formats"

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Co-authored-by: juanfra <juanfra@git.wordpress.org>
Co-authored-by: carolinan <poena@git.wordpress.org>
Co-authored-by: audrasjb <audrasjb@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>

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[jasmussen]

Thank you!

[audrasjb]

As a GTE and Locale Manager for fr_FR (and core committer) I approve this changes to be included post-String freeze.

[juanfra]

Thanks!