Fix documentation to match code (#23)

The type of the `Writer` field was changed from `*syslog.Writer` to
`io.Writer` back in commit de8e7b7, but
the README file still gives the old definition.

Author: fragglet

README.md

@@ -92,7 +92,7 @@ type Option struct {
 	Level slog.Leveler
 
 	// connection to syslog server
-	Writer *syslog.Writer
+	Writer io.Writer
 
 	// optional: customize json payload builder
 	Converter Converter