Update example of usage CallerMarshalFunc (rs#475)

There was changes in rs#457  pass program counter to CallerMarshalFunc.
Update example in README.md

README.md

@@ -411,7 +411,7 @@ log.Info().Msg("hello world")
 Equivalent of `Lshortfile`:
 
 ```go
-zerolog.CallerMarshalFunc = func(file string, line int) string {
+zerolog.CallerMarshalFunc = func(pc uintptr, file string, line int) string {
     short := file
     for i := len(file) - 1; i > 0; i-- {
         if file[i] == '/' {