br: fix the backup code example

[Oreoxmt]

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What is changed, added or deleted? (Required)

br: fix the backup code example from ./br restore full -u "${PD_IP}:2379" \ to ./br backup full -u "${PD_IP}:2379" \

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[Oreoxmt]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14103.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #14104.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #14105.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14106.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.2: #14107.