Fix validation contract documentation (#260)

Author: tsubery

CHANGELOG.md

@@ -10,6 +10,7 @@
 ### Bug fixes
 
 * Fix missing new_ostruct_member in Ruby 2.7 ([#255](https://github.com/railsconfig/config/pull/255))
+* Fix validation contract documentation ([#260](https://github.com/railsconfig/config/pull/260))
 
 ### Changes
 

README.md

@@ -321,7 +321,7 @@ class ConfigContract < Dry::Validation::Contract
 end
 
 Config.setup do |config|
-  config.validation_contract = ConfigContract
+  config.validation_contract = ConfigContract.new
 end
 ```