docs: remove unnecessary -itd from examples

[dvdksn]

- What I did

The -itd options, probably used to keep the tty container running in the background, is not necessary for these examples. Updated the examples to remove the unnecessary flags. Also, changed the example image from busybox to nginx.

Relates to:

• docker/docs#19699

- How I did it

- How to verify it

- Description for the changelog

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