Update README.md

[FranzBusch]

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.

[czechboy0]

Why not a struct? Most services I conform are actually either structs or actors, but rarely classes, as those then require manual locking.

[gjcairo]

Yeah I kind of agree that class may also be misleading.

Regardless of whether we keep class or change it to struct though, can we not add a comment saying that it actually doesn't matter whether the type is a class, an actor or a struct? If it's brought confusion in the past, let's make it super obvious.

[FranzBusch]

@czechboy0 @gjcairo updated the example