br: br backup command help gives incorrect default value for checkum

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

run 'br --help'

### 2. What did you expect to see? (Required)
--checksum flag is not marked as default true in the output

### 3. What did you see instead (Required)
it says it's true

### 4. What is your TiDB version? (Required)

8.5 and master