DOC: fix a failing doctest in DataFrame.to_dict
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Based on #22459. Fix the docstring for DataFrame.to_dict. I also updated `ci/doctests.sh`.
Hello @Moisan! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/Moisan/pandas/blob/ecdd114f75231936995e3c02b96b8686a414dfaf/pandas/core/frame.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=h1) Report
> Merging [#22827](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cf11f71e423be65e9c2ae83ea1bab8b69ee79ca4?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22827/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22827   +/-   ##
=======================================
  Coverage   92.14%   92.14%           
=======================================
  Files         170      170           
  Lines       51017    51017           
=======================================
  Hits        47011    47011           
  Misses       4006     4006
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.57% <ø> (ø)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22827/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.11% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=footer). Last update [cf11f71...64f6050](https://codecov.io/gh/pandas-dev/pandas/pull/22827?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Sorry, from looking at the diff now, I don't find this an actual improvement. Eg in the output

```
{'cat': {'legs': 4, 'wings': 0}, 'falcon': {'legs': 2, 'wings': 2}}
```

I really have to think hard "are legs/wings the columns or was it cat/falcon ?" to know what the structure is (`{index -> {column -> value}}` or `{column ->  {index -> value}}`)

I find this is easier to see from a glance with something like:
```
{'row1': {'col1': 4, 'col2': 0}, 'row2': {'col1': 2, 'col2': 2}}
``` thanks @Moisan 