tidb: fix placement doc (#7073)

[ti-chi-bot]

This is an automated cherry-pick of #7073

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What is changed, added or deleted? (Required)

fix the wrong placement command, since the usage like that：

Usage:
  pd-ctl config placement-rules rule-bundle save [flags]

Flags:
  -h, --help        help for save
      --in string   the file contains all group configs and all rules (default "rules.json")
      --partial     do not drop all old configurations, partial update

unknown shorthand flag: 'i' in -in=rules.json

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[TomShawn]

This branch does not need cherry-pick.