[docker] fix command in readme template

What this MR does / why does DuMux need it:

Issue #1433 (closed) mentioned that a ./ for executing running a docker container with a script is missing in 2 places in the according README template for the docker image.

MR !3950 (merged) only fixes one of both occurences. This MR fixes the other occurence.

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• do the test pipelines pass? (see guide on how to run pipelines for a merge request)