Correct casing of users.po in translation docs

[TerryKern]

The users.po file which is used to overwrite the translations provided by the plugin should be lowercase. Seeing as though translation files are generally lowercase in CakePHP in addition to the fact that as soon as you deploy to a Linux environment the casing will prevent translation.

CakePHP docs: https://book.cakephp.org/4/en/core-libraries/internationalization-and-localization.html#language-files

[steinkel]

thank you!