fix casing in environment variables sample

Author: lukecolburn

README.md

@@ -358,7 +358,7 @@ And the following configuration:
 ```ruby
 Config.setup do |config|
   config.use_env = true
-  config.env_prefix = 'Settings'
+  config.env_prefix = 'SETTINGS'
   config.env_separator = '__'
   config.env_converter = :downcase
   config.env_parse_values = true