DOC: Improve DataFrame.select_dtypes examples
Greetings,

This is a minor change in the ``DataFrame.select_dtypes`` doc.
- Add formatting (such as in `np.number`), to illustrate better it is a piece of code.
- Add decimal place to the third column of the example to proper address it is a float column (maybe in the past float numbers didn't show decimal places all the time, but now they do).

Thanks for reviewing this!
Let me know if I can help further more.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=h1) Report
> Merging [#19188](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8acdf801c501a0ce2ac14ddd676cd248f0f32180?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19188/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19188      +/-   ##
==========================================
+ Coverage   91.52%   91.54%   +0.02%     
==========================================
  Files         147      147              
  Lines       48775    48775              
==========================================
+ Hits        44639    44651      +12     
+ Misses       4136     4124      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.91% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.62% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19188/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.62% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19188/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=footer). Last update [8acdf80...4b2119a](https://codecov.io/gh/pandas-dev/pandas/pull/19188?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!