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)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.43%. Comparing base (4270341) to head (a80c7c4). Report is 569 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5214   +/-   ##
=======================================
  Coverage   61.43%   61.43%           
=======================================
  Files         298      298           
  Lines       20799    20799           
=======================================
  Hits        12777    12777           
  Misses       7109     7109           
  Partials      913      913