DOC: changes to use code-block declaration
- [ ] closes #19400
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks,

Looks like there was an extra file commited "doc/test.json". Could you delete it and then

```
git add doc/test.json
git commit -m "removed file"
git push
```

Then we'll get this merged. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=h1) Report
> Merging [#19400](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d3f7d2a666aa824e2df98083aa5c1fd9bb63252e?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19400/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19400      +/-   ##
==========================================
- Coverage   91.65%   91.62%   -0.03%     
==========================================
  Files         150      150              
  Lines       48726    48726              
==========================================
- Hits        44658    44646      -12     
- Misses       4068     4080      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19400/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=footer). Last update [d3f7d2a...9a78405](https://codecov.io/gh/pandas-dev/pandas/pull/19400?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
