system variables: correct the description of tidb_allow_fallback_to_tikv

[en-jin19]

What is changed, added or deleted? (Required)

Corrected the description of tidb_allow_fallback_to_tikv in "system-variables.md".

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• v3.0 (TiDB 3.0 versions)
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What is the related PR or file link(s)?

• This PR is translated from: system-variables: correct the description of tidb_allow_fallback_to_tikv docs-cn#7238
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• Delete files
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• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• qiancai

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[xuyifangreeneyes]

LGTM

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@xuyifangreeneyes: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

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[qiancai]

/remove-status LGT1
/status LGT2

[qiancai]

/merge

[ti-chi-bot]

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

Commit hash: 3d92839

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6940.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6942.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6943.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6944.