Update README.md

You cannot perform the not (`!`) operator on an `error`. It gives you `invalid operation: ! error`.

Tested in Go Playground: http://play.golang.org/p/jhlsHoS_gc
and locally on go1.2

README.md

@@ -10,7 +10,7 @@ This library is considered production ready.  Feedback and feature requests are
 		return myLookupKey(token.Header["kid"])
 	})
 
-	if !err && token.Valid {
+	if err == null && token.Valid {
 		deliverGoodness("!")
 	} else {
 		deliverUtterRejection(":(")