[FEATURE] Update text of duties description

[maradik]

Is your feature request related to a problem? Please describe.
I wanna change text of duties description for a rotation but the only way to do it is to re-create the rotation.

Describe the solution you'd like
Would be great to have something like this:
@dienstplan update-description <rotation name> <duties description>

[pilosus]

Related to p2. in the issue 65.
I'd suggest to keep things simple: instead of update-description use update. It will require to update both <list of user mentions> and <duties description>. But usually it's a not a big deal to copy-paste the list of mentions from the previous rota state, and it will spare time from implementing update-user-mentions later :-)

[pilosus]

@maradik done in the release 0.4.0.
Docker image is available as well.

I consider the issue to be resolved. In case of any problems, please file a new issue.