docs: fix french wrong howto target (#5556)

Completes review https://github.com/EbookFoundation/free-programming-books/pull/5554#pullrequestreview-763701519

Resolves https://github.com/EbookFoundation/free-programming-books/pull/5554#discussion_r716211068

HOWTO.md

-Read this in other languages: [Français](CONTRIBUTING-fr.md), [Español](HOWTO-es.md), [简体中文](HOWTO-zh.md), [हिन्दी](HOWTO-hi.md), [繁體中文](HOWTO-zh-TW.md), [Português (BR)](HOWTO.pt_BR.md), [فارسی](HOWTO-fa_IR.md)
+Read this in other languages: [Français](HOWTO-fr.md), [Español](HOWTO-es.md), [简体中文](HOWTO-zh.md), [हिन्दी](HOWTO-hi.md), [繁體中文](HOWTO-zh-TW.md), [Português (BR)](HOWTO.pt_BR.md), [فارسی](HOWTO-fa_IR.md)
 
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