tidb: update follower read guide

[glorv]

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• SunRunAway
• qiancai

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[ran-huang]

Rest LGTM

[qiancai]

/merge

[ti-chi-bot]

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

Commit hash: cb81532