Fix parameter name in help for az container create

[seanmck]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• The PR has modified HISTORY.rst with an appropriate description of the change (see Modifying change log).

Command Guidelines

• Each command and parameter has a meaningful description.
• Each new command has a test.

(see Authoring Command Modules)

[msftclas]

@seanmck,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[codecov-io]

Codecov Report

Merging #4208 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4208   +/-   ##
=======================================
  Coverage   70.04%   70.04%           
=======================================
  Files         475      475           
  Lines       29602    29602           
  Branches     4539     4539           
=======================================
  Hits        20736    20736           
+ Misses       7422     7418    -4     
- Partials     1444     1448    +4

Impacted Files	Coverage Δ
...ainer/azure/cli/command_modules/container/_help.py	100% <ø> (ø)	⬆️
...li-cloud/azure/cli/command_modules/cloud/custom.py	13.79% <0%> (ø)	⬆️
...dback/azure/cli/command_modules/feedback/custom.py	31.25% <0%> (ø)	⬆️
src/azure-cli-core/azure/cli/core/util.py	70.66% <0%> (ø)	⬆️
...nent/azure/cli/command_modules/component/custom.py	16.26% <0%> (ø)	⬆️

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[tjprescott]

We don't require/desire release notes for changes in help text.

[seanmck]

@tjprescott: Thanks for the review. I've reverted the change to the changelog.