How to fix the move code?

Question

Unity

I started to study Unity with a book, but I ran into a bug in the code of the same book. when turning the camera up or down, she thinks that I'm on a slope and I start to go forward or backward, as I guess, because of the physical value g 9.8 how to fix this bug and with an explanation of the correction I would like

using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Threading;
using UnityEngine;
[RequireComponent(typeof(CharacterController))]
[AddComponentMenu("Control Script/FPS Input")]
public class PlayerMoving : MonoBehaviour
{
    public float speed = 6.0f;
    public float gravity = -9.8f;
    private CharacterController _charController;
    void Start()
    {
        _charController = GetComponent<CharacterController>();
    }
    void Update()
    {
        float deltaX = Input.GetAxis("Horizontal") * speed;
        float deltaZ = Input.GetAxis("Vertical") * speed;
        Vector3 movement = new Vector3(deltaX, 0, deltaZ);
        movement = Vector3.ClampMagnitude(movement, speed);
        movement.y = gravity;
        movement *= Time.deltaTime;
        movement = transform.TransformDirection(movement);
        _charController.Move(movement);
    }
    }