fix: unknown command "ls" for "pouch images"

[tiny1990]

1.Describe what this PR did
fix doc
2.Does this pull request fix one issue?

NONE
3.Describe how you did it
source code build and ./pouch images ls
4.Describe how to verify it
NONE

[pouchrobot]

We found this is your first time to contribute to Pouch, @tiny1990
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[CLAassistant]

All committers have signed the CLA.

[fuweid]

Hi @tiny1990 , the pouch images is expected command. This command doesn't need the ls.

[codecov-io]

Codecov Report

Merging #620 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #620   +/-   ##
=======================================
  Coverage   13.58%   13.58%           
=======================================
  Files          65       65           
  Lines        3541     3541           
=======================================
  Hits          481      481           
  Misses       3010     3010           
  Partials       50       50

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[tiny1990]

@0x04C2 so i delete ls .

[fuweid]

Sorry, I make a mistake... Good catch!!!!

[tiny1990]

@0x04C2 a hello world pull request

[Letty5411]

@tiny1990 Could you change your commit message with title "fix: " ?

[tiny1990]

@Letty5411 change to refactor?

[fuweid]

Hi @tiny1990 , please use the following command to update your commit message:

git commit -s --amend

And git push -f origin master

[tiny1990]

@0x04C2 thanks

[Letty5411]

@tiny1990 change to fix is OK.

[Letty5411]

LGTM