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.