Update vc-demo-mars3d.md

[GreenBoy0526]

mars3d官网改了一些页面的地址

Please make sure these boxes are checked before submitting your PR, thank you!

• [ ] Make sure you follow VueCesium's contributing guide English | (中文).
• [ ] Make sure you are merging your commits to dev branch.
• [ ] Add some descriptions and refer to relative issues for your PR.

[zouyaoji]

hi @GreenBoy0526，非常感谢您的 PR ，但是您没有使用 PR 标题规则 格式，请及时修改， 谢谢！