docs: fix "Build docs" command in developers.rst

[wallberg]

Summary of changes

Fix the "Build docs" command in docs/developers.rst to, task docs:build, to match the actual task
in Taskfile.yml, task docs .

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• All tests and type checking are not passing, but I believe this is due to pre-existing issues.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.