Fix interpolation for "could not find config.ru" message

[bethesque]

"Could not find config.ru file at #{Pact.configuration.config_ru_path} Please configure the service provider app or create a config.ru file in the root directory of the project. See https://github.com/realestate-com-au/pact/blob/master/documentation/verifying-pacts.md for more information."

Line: https://github.com/realestate-com-au/pact/blob/866d603aefdc8a21b796712d5450344145c3a7b3/lib/pact/provider/configuration/service_provider_dsl.rb#L18