Manual of br integration tests is outdated

[RidRisR]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Follow the steps in br/tests/README.md

2. What did you expect to see? (Required)

Be able to run integration tests

3. What did you see instead (Required)

Errors becasue of incorrect instructions

4. What is your TiDB version? (Required)

[YuJuncen]

(I will change the type to an enhancement, since I guess we don't need to backport this.)