*: update tidb-server download URL (#611)

[ti-srebot]

cherry-pick #611 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In docs-dm repo:
git pr https://github.com/pingcap/docs-dm/pull/618

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/docs-dm.git pr/618:release-2.0-dc5f40247d80

---

What is changed, added, or deleted? (Required)

Which DM version(s) do your changes apply to? (Required)

• master (the latest development version, including v3.0 changes)
• v2.0 (TiDB DM 2.0 versions)
• v1.0 (TiDB DM 1.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): some comments from Document about quick-start-with-dm has incorrect info dm#1431

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@lance6716 you're already a collaborator in bot's repo.

[CharLotteiu]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CharLotteiu

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[CharLotteiu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6d9940a