Remove Deprecated methods in README.md

[sabi1125]

package main

import (
  "fmt"
  "io/ioutil"

  "github.com/h2non/filetype"
)

func main() {
  buf, _ := ioutil.ReadFile("sample.jpg")

  kind, _ := filetype.Match(buf)
  if kind == filetype.Unknown {
    fmt.Println("Unknown file type")
    return
  }

  fmt.Printf("File type: %s. MIME: %s\n", kind.Extension, kind.MIME.Value)
}

io/ioutil's ReadFile is Deprecated since go version v1.16.

Diagnostics:
1. ioutil.ReadFile is deprecated: As of Go 1.16, this function simply calls [os.ReadFile]. [default]
2. Call of ioutil.ReadFile should be inlined [inline_call]

It would be better to use the os.ReadFile in the example instead.