Update --ssh desc

[glkappe]

What problem does this PR solve?

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Once this PR has been reviewed and has the lgtm label, please assign kaaaaaaang for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 50.88%. Comparing base (9e11d0e) to head (29644e6).

:exclamation: Current head 29644e6 differs from pull request most recent head bec394d. Consider uploading reports for the commit bec394d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2380 +/- ## ========================================== - Coverage 56.05% 50.88% -5.17% ========================================== Files 329 328 -1 Lines 35040 35017 -23 ========================================== - Hits 19639 17815 -1824 - Misses 13064 14877 +1813 + Partials 2337 2325 -12 ``` | [Flag](https://app.codecov.io/gh/pingcap/tiup/pull/2380/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [tiup](https://app.codecov.io/gh/pingcap/tiup/pull/2380/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `33.59% <ø> (-0.05%)` | :arrow_down: | | [unittest](https://app.codecov.io/gh/pingcap/tiup/pull/2380/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more.

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