doc: change the form name of upload example code (#3076)

README.md

@@ -385,7 +385,7 @@ func main() {
 	router.MaxMultipartMemory = 8 << 20  // 8 MiB
 	router.POST("/upload", func(c *gin.Context) {
 		// Single file
-		file, _ := c.FormFile("Filename")
+		file, _ := c.FormFile("file")
 		log.Println(file.Filename)
 
 		// Upload the file to specific dst.
@@ -417,7 +417,7 @@ func main() {
 	router.POST("/upload", func(c *gin.Context) {
 		// Multipart form
 		form, _ := c.MultipartForm()
-		files := form.File["Filename[]"]
+		files := form.File["upload[]"]
 
 		for _, file := range files {
 			log.Println(file.Filename)