Improve error message for i18next key format rule

[nathanielkohl83]

• Updated the error message in the i18next key format rule to provide clearer guidance on valid key formats.
• Clarified that the module and feature parts of the translation key must use only lowercase letters (a-z) and hyphens (-).
• Highlighted that only the subkey part may include uppercase letters, underscores (_), or periods (.).
• Replaced the misleading all-caps example with a more accurate and valid example: my-module.my-feature.my_key.
• These changes aim to reduce confusion and save time for developers when debugging key format errors.

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