Add logger argument in the README (#141)

# Motivation
We were missing a `logger` argument for the `ServiceGroup` in the README.

# Modification
Adds the missing `logger` argument and renames a file

# Result
Fixes #139

Author: FranzBusch

README.md

@@ -56,7 +56,8 @@ struct Application {
 
         let serviceGroup = ServiceGroup(
             services: [service1, service2],
-            configuration: .init(gracefulShutdownSignals: [.sigterm])
+            configuration: .init(gracefulShutdownSignals: [.sigterm]),
+            logger: logger
         )
         try await serviceGroup.run()
     }