fix containerd-plugins-namespace typos

[Peztel]

- What I did add 's' to 'containerd-plugin-namespace' cli arg

- How I did it add 's' to the document

- How to verify it attempt to start docker engine service. verify that it does not fail because of 'container-plugin-namespace'

- Description for the changelog

fix cli arg typo

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

[codecov-commenter]

Codecov Report

Merging #3216 (11d98e5) into master (2776a6d) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3216   +/-   ##
=======================================
  Coverage   58.57%   58.58%           
=======================================
  Files         299      299           
  Lines       21502    21502           
=======================================
+ Hits        12595    12597    +2     
+ Misses       7984     7983    -1     
+ Partials      923      922    -1