Update README.md

Typo it seems - except if I still didn't understand go...

README.md

@@ -317,7 +317,7 @@ type MyJSONFormatter struct {
 
 log.SetFormatter(new(MyJSONFormatter))
 
-func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
+func (f *MyJSONFormatter) Format(entry *Entry) ([]byte, error) {
   // Note this doesn't include Time, Level and Message which are available on
   // the Entry. Consult `godoc` on information about those fields or read the
   // source of the official loggers.