[Resource Editor] Change description for i18n paragraph

Internationalization files uploaded here will be available on the Nuxeo user interface. You must use proper i18n file name format: messages_[COUNTRY].properties.

Currently the text indicates a suffix with capital letters, which makes the user create a file named messages_EN.properties for english resources.
But this file won't be merged with the default file messages_en.properties if the user is running a case-sensitive OS
As a consequence, no new translation available.

Maybe we need to change the description or give a sample