fix golint issues in rest (#529)

rest/handler/maxconnshandler.go

@@ -8,6 +8,7 @@ import (
 	"github.com/tal-tech/go-zero/rest/internal"
 )
 
+// MaxConns returns a middleware that limit the concurrent connections.
 func MaxConns(n int) func(http.Handler) http.Handler {
 	if n <= 0 {
 		return func(next http.Handler) http.Handler {