Update README.md

[jesperhodge]

The commit d5e5171 removed the make command docker_build from the Makefile.

When setting up the repo, the first step in the README is to run docker_build, which results in a confusing error.
Skipping that step works just fine, so I'm removing it from the README.

[bcitro]

Thanks for fixing this Jesper! You'll just need to update your commit message to conform with Conventional Commits to get that linter check to pass. In this case, you just need to add the type of the commit to the beginning, which should be docs:.

More info here: https://open-edx-proposals.readthedocs.io/en/latest/best-practices/oep-0051-bp-conventional-commits.html