📚 Doc: Fix code snippet indentation in /docs/api/middleware/keyauth.md

Removes an an extra level of indentation in line 51 of
`keyauth.md` [here](https://github.com/gofiber/fiber/blob/v2/docs/api/middleware/keyauth.md?plain=1#L51)

Author: grivera64

docs/api/middleware/keyauth.md

@@ -47,7 +47,7 @@ func main() {
 		Validator:  validateAPIKey,
 	}))
 
-		app.Get("/", func(c *fiber.Ctx) error {
+	app.Get("/", func(c *fiber.Ctx) error {
 		return c.SendString("Successfully authenticated!")
 	})