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

[ti-srebot]

cherry-pick #5859 to release-3.1
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git pr https://github.com/pingcap/docs-cn/pull/6009

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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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[TomShawn]

/merge

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

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Commit hash: 9ece0d8