Update the readme, since Duration is deprecated (#1015)

* Update the readme, since Duration is deprecated

Chance to Expiration

* Update README.md

* Update README.md

middleware/limiter/README.md

@@ -30,12 +30,13 @@ After you initiate your Fiber app, you can use the following possibilities:
 app.Use(limiter.New())
 
 // Or extend your config for customization
+
 app.Use(limiter.New(limiter.Config{
 	Next: func(c *fiber.Ctx) bool {
 		return c.IP() == "127.0.0.1"
 	},
 	Max:          20,
-	Duration:     30 * time.Second,
+	Expiration: 30 * time.Second,
 	Key:          func(c *fiber.Ctx) string {
 		return c.Get("x-forwarded-for")
 	},
@@ -60,10 +61,10 @@ type Config struct {
 	// Default: 5
 	Max int
 
-	// Duration is the time on how long to keep records of requests in memory
-	//
+	// Expiration is the time on how long to keep records of requests in memory
+	// 
 	// Default: time.Minute
-	Duration time.Duration
+	Expiration time.Duration
 
 	// Key allows you to generate custom keys, by default c.IP() is used
 	//
@@ -103,4 +104,4 @@ var ConfigDefault = Config{
 		return c.SendStatus(fiber.StatusTooManyRequests)
 	},
 }
-```
+```