fix: docker login autocomplete for zsh

[rhnvrm]

- What I did

Changed --user to --username

- How I did it

Edited zsh autocomplete file

- How to verify it

You can use zsh and press tab after typing docker login --

- Description for the changelog

Fix for docker login username flag in zsh autocomplete

- A picture of a cute animal (not mandatory but encouraged)

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:rhnvrm/cli.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Codecov Report

Merging #1948 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1948   +/-   ##
=======================================
  Coverage   56.72%   56.72%           
=======================================
  Files         310      310           
  Lines       21800    21800           
=======================================
  Hits        12367    12367           
  Misses       8518     8518           
  Partials      915      915

[thaJeztah]

LGTM

@vdemeester ptal

[vdemeester]

LGTM 🐯