Settings export "Save as file" should reference JSON type

[ibennetch]

In the Settings area, we can "Export as file" but it doesn't specify what type of file that is. I think it should say "Export as JSON file".

This text is used twice, as Export_asfile_name in libraries/classes/Config/Descriptions.php and in prefs_manage.php. We need to verify the use of each of those before making the change.

This probably needs to be a dirty translation change.

[ibennetch]

libraries/classes/Config/Descriptions.php references the settings -> export setting to save the export as a file, so this is fine as-is.