Fix description of filter flag in prune commands

[apkd]

- What I did
Fix description of filter flag in prune commands.

- How to verify it
None of the following work demonstrating the issue:

user@ubuntu:~$ docker image prune -f "until=24h"
"docker image prune" accepts no arguments.
See 'docker image prune --help'.

Usage:  docker image prune [OPTIONS] [flags]

Remove unused images
user@ubuntu:~$ docker volume prune -f "until=24h"
"docker volume prune" accepts no arguments.
See 'docker volume prune --help'.

Usage:  docker volume prune [OPTIONS] [flags]

Remove all unused volumes
user@ubuntu:~$ docker network prune -f "until=24h"
"docker network prune" accepts no arguments.
See 'docker network prune --help'.

Usage:  docker network prune [OPTIONS] [flags]

Remove all unused networks
user@ubuntu:~$ docker container prune -f "until=24h"
"docker container prune" accepts no arguments.
See 'docker container prune --help'.

Usage:  docker container prune [OPTIONS] [flags]

Remove all stopped containers
user@ubuntu:~$ docker system prune -f "until=24h"
"docker system prune" accepts no arguments.
See 'docker system prune --help'.

Usage:  docker system prune [OPTIONS] [flags]

Remove unused data

Correct output when the proper flag is used:

user@ubuntu:~$ docker image prune --filter "until=24h"
WARNING! This will remove all dangling images.
Are you sure you want to continue? [y/N] n
Total reclaimed space: 0B

- Description for the changelog
Fix description of filter flag in prune commands. The "-f" flag is an alias for --force, not --filter (as correctly stated at the top of each documents). The system_prune.md didn't have this error.

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

[codecov-io]

Codecov Report

Merging #774 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #774   +/-   ##
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  Coverage   53.46%   53.46%           
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  Files         218      218           
  Lines       14642    14642           
=======================================
  Hits         7829     7829           
  Misses       6327     6327           
  Partials      486      486

[dnephin]

LGTM

[vdemeester]

LGTM 🦁