Minor typo fix in run documentation

[ScottBrenner]

- What I did
Typo fix for docker run's --volume documentation.

- Description for the changelog
Typo fix for docker run's --volume documentation.

- 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:ScottBrenner/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.

[thaJeztah]

LGTM, thanks!

[codecov-io]

Codecov Report

Merging #1434 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1434   +/-   ##
=======================================
  Coverage   54.22%   54.22%           
=======================================
  Files         289      289           
  Lines       19359    19359           
=======================================
  Hits        10498    10498           
  Misses       8185     8185           
  Partials      676      676