Update use-br-command-line-tool.md (#5859)

[ti-srebot]

cherry-pick #5859 to master
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---

br 的validate 子命令，需要使用tail -n1 将时间戳赋值给变量，否则会将Starting component br: /home/iass_user/.tiup/components/br/v4.0.10/br validate decode --field=end-version -s local:///data/nfs_test/backup-ceshi 423818981889802243 执行过程以及结果赋值。

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[ti-srebot]

@liufugen-yewu please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs-cn/invitations

[TomShawn]

/lgtm

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[ti-chi-bot]

@TomShawn: /merge in this pull request requires 2 /lgtm.

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/merge

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[TomShawn]

/label require-LGT1

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 7c7c64e

[TomShawn]

/close

[ti-chi-bot]

@TomShawn: Closed this PR.

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/close

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[TomShawn]

/reopen

[ti-chi-bot]

@TomShawn: Reopened this PR.

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/reopen

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