Update README.md (#197)

The readme shows an actor implementing the `Service` protocol. This has
lead to some people thinking that only actors can do this so let's
switch this to a `class` which is the usual type conforming to
`Service`.

README.md

@@ -66,7 +66,8 @@ the configured signals and trigger a graceful shutdown on each service.
 import ServiceLifecycle
 import Logging
 
-actor FooService: Service {
+// A service can be implemented by a struct, class or actor. For this example we are using a struct.
+struct FooService: Service {
     func run() async throws {
         print("FooService starting")
         try await Task.sleep(for: .seconds(10))