fix: docstring for cli/command: ContainerFormat.CreatedAt

[lentil32]

fixes #2413

- What I did

Correct documentation for CreatedAt
As @Benehiko suggested in #4765 (comment), didn't add CreatedAtTimestamp.

- How I did it

1. Referred to https://pkg.go.dev/time#Time.String
2. Used commenting style as for DisplayablePorts in same file

- How to verify it

N/A, or refer to func TestContainerContextWriteJSON(t *testing.T) { in formatter/test_container.go

- Description for the changelog

cli/command: fix docstring for ContainerFormat.CreatedAt

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

[Benehiko]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.73%. Comparing base (6b74160) to head (0b9d582).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5415   +/-   ##
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  Coverage   59.73%   59.73%           
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  Files         345      345           
  Lines       23394    23394           
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  Hits        13974    13974           
  Misses       8450     8450           
  Partials      970      970