Update stats.md add example json output

[diepes]

Add an example producing json output.

- What I did
Add another docker stats format example

- How I did it
Edit in github

- How to verify it
ran on server

- Description for the changelog

Add docker stats example with json output

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

[codecov-commenter]

Codecov Report

Merging #3378 (0d44008) into master (a45974a) will increase coverage by 1.66%.
The diff coverage is n/a.

❗ Current head 0d44008 differs from pull request most recent head a1204a5. Consider uploading reports for the commit a1204a5 to get more accurate results

@@            Coverage Diff             @@
##           master    #3378      +/-   ##
==========================================
+ Coverage   56.35%   58.02%   +1.66%     
==========================================
  Files         304      302       -2     
  Lines       26830    21761    -5069     
==========================================
- Hits        15121    12626    -2495     
+ Misses      10789     8214    -2575     
- Partials      920      921       +1     

[thaJeztah]

Thank you for contributing! It appears your commit message is missing a DCO sign-off,
causing the DCO check to fail.

We require all commit messages to have a Signed-off-by line with your name
and e-mail (see "Sign your work"
in the CONTRIBUTING.md in this repository), which looks something like:

Signed-off-by: YourFirsName YourLastName <yourname@example.org>

There is no need to open a new pull request, but to fix this (and make CI pass),
you need to amend the commit(s) in this pull request, and "force push" the amended
commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs
to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found
in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "yourname@example.org"

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff

   If your pull request contains multiple commits, either squash the commits (if
   needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Sorry for the hassle (I wish GitHub would make this a bit easier to do), and let me know if you need help or more detailed instructions!

[diepes]

@thaJeztah hi, added the required sign-off

[thaJeztah]

Thank you for fixing the sign-off! (sorry for the hassle 😓)

I spotted one minor issue; let me do a quick fix up

[thaJeztah]

I see there's double back-ticks around json. GitHub seems to be fine with that, but let me quickly fix that up and push to this branch (in case docs.docker.com doesn't like it when generating the HTML) 😅

[thaJeztah]

LGTM