go.net/proxy: fix typo

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7301052

proxy/per_host.go

@@ -9,7 +9,7 @@ import (
 	"strings"
 )
 
-// A PerHost directs connections to a default Dailer unless the hostname
+// A PerHost directs connections to a default Dialer unless the hostname
 // requested matches one of a number of exceptions.
 type PerHost struct {
 	def, bypass Dialer