Correct unclear description in feature-online-ddl

[shichun-0415]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

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

Correct unclear description in feature-online-ddl

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

Since December 23, 2021, the TiDB DM documentation in the master branch has been merged to TiDB documentation by #8042 and #7317. To create a PR for TiDB DM versions later than v5.3, either go to TiDB English documentation repository or Chinese documentation repository.

• v5.3 (TiDB DM 5.3 versions)
• 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 DDLs still need to be copied." is unclear for me #886

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-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lichunzhu

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[lichunzhu]

LGTM. Great job!

[hfxsd]

Rest LGTM

[hfxsd]

/merge

[ti-chi-bot]

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

Commit hash: 85fc8cb