Fix ScalaPbJsonMarshaller class name in doc (#4434)
Fix ScalaPbJsonMarshaller class name in doc (#4434)

Motivation:

Fix `ScalaPBJsonMarshaller` to `ScalaPbJsonMarshaller` in the ScalaPB integration documentation

Modifications:

- Just update code examples in the documentation
 
Result:

- No impact except for correcting the documentation

kojilin/armeria,trustin/armeria,line/armeria,trustin/armeria,trustin/armeria,trustin/armeria,line/armeria,line/armeria,kojilin/armeria,trustin/armeria,kojilin/armeria,trustin/armeria,line/armeria,kojilin/armeria,line/armeria,kojilin/armeria,kojilin/armeria,line/armeria