docs: use --quiet in example to simplify output

[thaJeztah]

• relates to https://github.com/docker/cli/pull/5090

The output showed the Alpine version that was used for the example, which can get outdated and distracts from the example steps.

Use --quiet to reduce the output, and to reduce maintenance (i.e., no need to keep the output updated with current versions).

- 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 61.34%. Comparing base (421e3b5) to head (82ed39e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5091 +/- ## ======================================= Coverage 61.34% 61.34% ======================================= Files 298 298 Lines 20706 20706 ======================================= Hits 12703 12703 Misses 7103 7103 Partials 900 900 ```